Sometimes the motivation for adopting a lovebird is really to adopt two so you can have a pair. If you want two birds so they can keep each other company, it's not necessary to have one of each gender. But if you want them to have baby lovebirds, you need to know how to tell a male from a female to guarantee you have what it takes to get babies.
Sometimes You Can ...
Some breeds of lovebirds are sexually dimorphic, meaning that the males and females have obvious differences in their physical characteristics. In Madagascar lovebirds, for instance, a male has a gray head and neck while the female is completely green, no gray markings at all. Another example is the red-faced lovebird. The males of this type are the ones with the red faces, while the females' faces tend to be more orange than red. Male red-faced lovebirds have light blue on their hind-ends and have black flight wings.
... Sometimes You Can't
Some types of lovebirds look exactly alike, male and female. Lovebirds like the masked lovebird, the peach-faced lovebird and Fischer's lovebird are nearly impossible to tell one gender from the other.
The Vet Can Help
If you have a pair of identical lovebirds, your vet can help figure out if you have one of each or a truly matched single-gender set. The easiest and least stressful way to determine the gender of a lovebird is through a blood test. Your vet will take a small sample of blood from your birds and send them to a lab for DNA testing that determine which chromosomes are present. Another way is for your avian vet to do a surgical examination of your birds' reproductive system. Since birds' sex organs are inside their bodies, the vet will have to anesthetize your birds and make an incision to insert an optical fiber to take a look inside.
Maybe you're simply curious about the genders of your lovebirds and it really doesn't matter to you what they are. If that is the case, there's no need to traumatize your birds with a surgical procedure or take blood from them to test their DNA. You can always look at telltale physical characteristics and behavior to make a guess. In many monomorphic birds, the females are just a bit larger than the males, and sometimes the females' beaks are larger. A clue you can't miss is that females are the only ones who lay eggs. They also are more aggressive and territorial. Female lovebirds will chew up nesting material like paper and stash it in the feathers near the derriere to carry to the nesting site. Some males chew up nesting material, but they don't tuck it away in their feathers.