It depends on the species, as some are sexually dimorphic (visual difference between male and female). Also, if the bird carries a sex-linked mutation, it can be visually sexed. For example in Peach-faced Lovebirds, a female ino and a male pallid will create pallid females and pallid ino males. These are sex-linked mutations. The Lovebird species which are sexually dimorphic are the Madagascar, Abyssinian, and Red-headed Lovebird. Non sexually dimorphic species are the Peach-faced, Masked, Fischer's, Lilian's, and Black-cheeked Lovebirds. Current sex-linked mutations in Peach-faced Lovebirds are Cinnamon, Ino, Pallid, and Opaline. In households with single Lovebirds that are not sexually dimorphic, the only way to know without DNA testing or probing, is whether or not any eggs are laid. Though that is not a guarantee as many females do not lay eggs if there is improper care. Females do tend to be more aggressive, males tend to be more vocal as well, though that isn't always the case. The pelvic bone in females who have laid eggs previously will be split, whereas a males will be fused. Young females do not always have a space, though their pelvic bone is more rounded. Without a comparison it is not reliable. The best way to be sure is through DNA testing, though if you don't plan to breed, it shouldn't really matter.