How do I recognize signs of recognition when I call my bf by his screen name on a dating site in person?

Well if he is lying he will probably not make eye contact with you, if you notice he's looking at everything else but you. If you notice him sweating or turning red in the face that would be a good indication because he will feel that adrenaline, the fight or flight process. If he starts stuttering or says nothing at all that's because he is thinking of what to say that you may believe. Hopefully he will just come clean at least give you that respect. The odds are against that I wish you the best and good luck.