How can you tell the difference between a guy who is just infatuated, and a stalker?

If he was a stalker he would know more things about you like where you live,phone and other things he may be infatuated with you and seems to find you at your work and says to you complements either way if you don't like it you can give him a warning and tell a male friend to talk to him to stay away from you and if he doesn't back out call the police