Women generally know when men are flirting with them. However, when the man is married it can be difficult to know if he is being friendly or flirting. While it may be considered taboo, his flirting may mean he's just being friendly. Learn to recognise the signs of flirting so that you know how to react to his behaviour.
Observe his body language. For example, if he intently looks into your eyes and smiles a lot while talking to you, he is likely flirting. If he appears to be checking out your physical appearance he may be flirting. A nervous demeanour can be an indicator that he's concerned about saying or doing something inappropriate.
Determine whether or not his comments are intended to make you laugh. Humour is often a part of flirting. If the humour is of a sexual or intimate nature, the odds are even greater that he's flirting.
Gauge comparisons to his wife. Comments like, "I like short skirts. My wife wears mostly long skirts, but I enjoy the beauty of female legs. That skirt really shows off your legs." This kind of comment is flirty because he is giving you a compliment, while also making a slightly, negative comparison to his wife.
Notice if he begins to share with you personal information. For example, if he is working out more and makes it a point to bring it up in conversation with you--but not necessarily anyone else--he is likely fishing for a compliment and some extra attention. He may even make comments like "I've been working out more lately, in the event that I find myself on the market again, I want to be able to attract women as beautiful as you." With this type of statement, he lets you know he notices you, pays you a compliment and touts his physical attributes.