Check his ring finger. It may seem obvious, but this is the first thing you should do to determine if a man is single. Look for a ring or the tan line from a ring. That's a dead giveaway if he removed his wedding band.
Assess the situation. If he appears to be out with some buddies on the prowl for ladies, it's likely that he's single. If he's hanging back and seems more reserved, he may be in a relationship.
Trust your intuition and your gut. If the man is married you'll pick up on hints, at least subconsciously.
Watch what he's watching. Single men are usually more likely to look around the room for women. There's a sense of availability and openness about them. Unavailable men may be more reserved.
Separate the single men from the men who are looking to cheat on their wives or girlfriends. This isn't the easiest task.
Find out more about the guy. Figure out who he hangs out with. If he's flirting just with you, it's likely he's single and interested. But, if he's flirting with a ton of women he may be a guy out to cheat on his woman.
Ask a friend if the man is single. It's not the most clandestine way of figuring it out, but it saves you a lot of time wondering.