Ever heard of the 3 second rule. It works much better than constant psycho staring.

If you see a female you like, it's good to assume if she's seen you she could be at least mildly physically attracted. The longer you wait, trying to have a stare contest or think of an opener, the more her anticipation and interest goes down. If she mentioned a boyfriend out of nowhere that can be a Shit Test. At that point it means knock her off guard with something clever. A neg like "Stop bringing up excuses, your imaginary bf isn't part of our conversation" Flip the situation make her look like the out of touch person. Keep on your toes, don't fail the shit testing.