It is impossible to sex young Lovebirds without a DNA test. Mature birds over one year can be pelvically sexed.
Secure the lovebird in one hand, laying on its back in your hand;
Feel just below the legs for two small bumps; these are the pelvic bones;
Females will have a significant gap between these bumps to allow eggs to pass and a male will usually have two bumps close together or be touching.