Every person and situation may be different, but there are some universal signs to know if a guy really likes you or if he's just plain nice.
He stares at you. If you find him looking your way, see if he's really staring at you or merely looking your way, or maybe cross-eyed. Maybe he's a credit investigator or something. Try to find out.
He asks personal questions. Asking the time or the date does not count unless he asks several times in an hour. If he asks questions like, "What do you like for your birthday?" or "Are you looking for a boyfriend now?" it could be a sign, unless he has a geeky brother and you could be in for a set-up.
He compliments you. If he says, "You remind me of my mother", take it as a compliment unless he hates his mother, in which case he may have emotional problems. When he says, "You don't need to diet, you look great", especially if you're 4'11 and 200 lbs, then it's a significant compliment.
He does extra helpful things. If he helps you finish your sandwiches, think again. Maybe he's only interested in your sandwich. However, if you needed to submit a report and he doesn't mind working until 3am and still has to go home in a shanty town where you get shot for smiling, then this guy is not just in love... He's crazy.
And you're crazier for not knowing he likes you.