If you're on regular speaking terms or are acquaintances who see each other frequently it should not be difficult to mention an event or whatever that's taking place and casually ask him if he'd like to check it out with you.
On the other hand if he doesn't know who you are and you have never established any rapport whatsoever then don't bother.
Odds are you probably don't know whether or not he is seeing someone, he has never flirted with you, or he's possibly gay. Not many people appreciate a "stranger" asking them out.
Lastly you have to consider the (age/maturity) of the guy. Some guys especially young ones are always open to accepting "low hanging fruit". These are the guys who tell their friends: "This chick came on to me the other day so I know I can tap that anytime I want." In other words he has NO interest in the woman but he'll take a "freebie" if it comes his way.
If a man is around you and has never flirted with you there is a good chance that he does not see you as dating/relationship material. On the other hand he may mistakenly believe (you) are unavailable. I knew a woman who wore her deceased grandmother's ring on her left hand and most guys thought she was married. In other instances women give off a vibe that they're unavailable. They seldom smile or make eye contact with men. Spend more time around him.