Body language, including facial expression, is one way you can tell things about a person that he might not even know. Then again, maybe he does.
He's a Comedian
Some people wink after making a joke or sarcastic remark (or if somebody else makes one). If a guy winks at you, he's making the extra effort to establish a personal understanding ("YOU get me, right?"). In this scenario, he is just trying to be friendly. It's who he is.
He Likes You
Read the rest of his body language or actions. Did he just offer you a drink? Many guys don't want to come off as overbearing in a social situation. Making general eye contact is a good subtle indicator that he has at least some interest in you. If he puts a hand on your shoulder while telling a joke, winks and walks away, he probably has romantic inclinations toward you.
Or Maybe It Was Nothing
There's always the possibility it was just dust in his eye. Some people also develop nervous tics in social situations. Keep watching him; a nervous guy might want you to make the first move.
In Latin American countries, winking is almost assuredly a romantic signal. Throughout Asia, though, it is widely considered rude.