Whether you choose one Love Bird for a pet, or a pair, you will want to determine the sex of the bird. Generally, the hen has a broader head, shoulders, and pelvic span than does the cock. The first two characteristics can be seen, the third, felt.
to feel the pelvic area, hold the bird with its back against your palm, with your thumb and little finger capturing the wings against the body. The head will protrude between your second and third fingers. This leaves the belly area clear for you to insert your finger between the bird’s legs. You should be able to feel two points of bone just above the tail. If the two points are extremely close together, the bird may be a male. If you can almost insert your little finger between the points, it may be a female. You have a 50% chance of being right!

 DNA sexing involves taking a couple drops of blood from the bird (or you can send a blood feather instead), dropping the blood on a small card, and sending the card through the Postal System to a Lab that processes the DNA.    

Sometimes you can sex birds by behavior, however, I have not found this process a viable alternative for sexing lovebirds.  Lovebirds get their name because they are so loving/affectionate toward one another.  Even 2 males together or 2 females together behave in this manner.
 There is one thing you can try, but it is not reliable or accurate.  In fact, even lovebird breeders often have a hard time using this method with as much experience as they have.  What you can do is to feel the bird's cloaca...the cloaca is the bird's vent (where the bird poops).  By feeling I mean you'll have to hold the bird with/in one hand (keep the bird right side up) and feel the vent with one of your fingers from your other hand.  Take some time to feel the structure of the bones (there will be a bone on each side), whether the bones easily spread apart/are flexible when you put a small amount of pressure on the area (this sounds gross, but press gently on the vent as if you might put your finger inside the bird).  A female's cloaca bones will be farther apart than a male's (in order to pass an egg).  The cloaca bones will spread easily when pressure is applied and there should not be a "point" to the bones on either side (the end of the bones should be smooth/rounded...not "pointy").  In other words, a female's will be shaped like the bottom of the letter U (rounded)...a male's will be shaped like the bottom of the letter V (pointed).       

In a male bird, the vent bones will not spread (or won't spread as far as a females) when you apply a little bit of pressure to the area, and you will notice that the vent bones have a definite point to them on either side.  You may have to go back and forth with the birds a couple of times before you become familiar with the physiology of the area.  You might even want to become familiar with the vent area using some books or information/pictures from the internet so you have some knowledge as to what you're feeling when you examine your birds.   

Like I stated, the above is not a reliable method of determining sex in any bird.  If you have a male and female, you should notice some difference however.  If you have more than 2 lovebirds, you could house them all in the same cage and let them pair up themselves.  If you only have 2 lovebirds, put up a nestbox and see if one (or both) lay eggs (females can lay eggs without a male being present, but the eggs won't be fertile).  

Vitamins/minerals/supplements:  When I first started out keeping birds many years ago, I bought all types of special bird vitamins/supplements, etc., trying to ensure my bird's were in the best of health.  I spent a small fortune and my birds weren't as healthy as they should be.  Not to say bird vitamins/supplements aren't effective...they are.  But I've found through the years that the best thing you can do for your bird(s) is to ensure they have adequate housing, human love/attention/interaction, enough exercise, activities/items (toys, etc.) that stimulate their minds (birds are very intelligent and require mental stimulation...budgies included), fresh water daily, proper lighting, good/clean/fresh air, and proper diet.  I prefer/recommend a more natural way of keeping birds healthy, supplementing as necessary when/where the bird(s) need it.  Since the experts don't know exactly what is a proper diet for birds, variety is the key.  Another reason for variety is that birds don't eat everything we give them when we feed them (a lot is lost to the cage bottom or house floor).  Parrots will eat something one day, and throw it to the floor the next day (depending on what else is in the bowl).  Therefore, we have to provide a good variety in different forms (for example, I have a bird that won't eat fresh bananas, but loves dried bananas) to ensure we meet their nutritional needs.  Once you know your birds, you'll know what they'll eat or not, and you can build their proper diet around this knowledge.  

With all that said, I believe we should keep fresh, good quality pelleted/extruded food in our bird's cages 24 hours per day, fresh water daily (or more often if needed for those "soup" makers!).  On a weekly basis, we should provide mostly vegetables (dark green and orange are best), grains (not same as seed), fruit (fruit is mainly water), legumes (cooked beans), greens (kale, mustard greens, but not lettuce, etc.), pasta/noodles (no sauce), cooked brown rice, etc., etc.  Seed should be held to minimum and used as a treat/reward/positive reinforcement, etc (seed is candy to birds).  And all birds love corn bread!  Veggies should be raw and clean...same for fruit (no citrus seeds).  Frozen only if fresh isn't available, and stay away from canned items as much as possible (too much salt, nutrition cooked out).  Grains, such as whole grain breads, nutritious dry sugarless cereals, etc.  Weekly basis meaning at the end of 7 days, your bird(s) diet (it's consumption, not what you provide) should have consisted of mostly veggies, some fruit, grains, legumes, etc....doesn't mean you have to provide all these items every day!  Greens might only be given once or twice a week, same with pasta, rice, etc.  A good, rounded, nutritional, varied diet at the end of every 7 days.  Keep in mind though that the size of food provided to a bird has to fit the size of the bird's beak (not the size of the bird).  In other words, for parakeets/budgies, you should probably shred their food items or prepare a mash-type mixture (cooked brown rice with green peas, shredded carrots.......).  Birds won't eat what they don't recognize, so keep offering and they will eventually try the new food.

To the point of your question...at the end of the week, if your bird(s) hasn't consumed a good variety of food or what you feel he should have for proper nutrition, then you should supplement for what s/he is lacking.  Pelleted/extruded food is normally "enriched" with vitamins/minerals, which is why I recommend this type food be available 24 hours per day.  A bird (particularly a breeding bird) might need supplemental calcium (with the proportional amount of Vit D included and proper lighting).  I always keep cuttlebones and mineral blocks in bird cages (helps keep beaks trimmed, too).  Nekton produces some good vitamins/minerals for birds.  

The goal is to PREVENT illness/disease.  Once a bird gets sick, it's often hard to cure them because most people don't recognize illness in a bird early enough.  I supplement almost daily (at least a few times per week) with herbal type vitamins/minerals to boost the immune system to prevent.  Once a bird is ill, I medicate for a specific illness.  I mix bottled powder or I open capsules from bottles and mix in or sprinkle on or bake in food.  I've had much success with Spirulina.  I also use Echinacea.  I've used green tea in drinking water.  I've also used Vanodine in drinking water.  I'm currently experimenting with Elderberry juice as a preventative and cure for certain illnesses.  Garlic juice and apple cider vinegar are great for preventing a lot of illnesses (on the food or in their water).  So many natural items I've tried successfully and/or used over the years!  Remember that illness/disease is generally spread through drinking water or feces...keep water fresh (and water container disinfected...use water bottles for best results) and clean and birds away from droppings as much as possible.