Typically to determine the sex of a parrot today, they can be surgically sexed by a veterinarian or more preferably, through DNA testing via a drop of blood sent to a lab.
female lovebirds makes strips
male lovebirds make confetti
With Lovebirds, although unscientific, you can perform a simple test. Throw a piece a newspaper into the bird cage, if the Lovebird makes confetti, it is a boy, if it makes long strips, like feathers and possibly tries to tuck into its feathers, then it is a girl. If they are too young, it may not work.