It shouldn't move at all, some people say you can feel their pelvic bones to be able to tell male or female based on how wide set they are but that has a lot more to do with age and if the bird is in breeding condition. The only real ways to tell male vs. female in lovebirds is if they lay an egg or you have them DNA tested. It'd probably be a good idea to get them tested that way you know to watch out for issues like egg binding and such.
