First you should ask men specifically (those that will tell you the truth are real men and want to be helpful)!
You can tell by a change in mannerisms, body language towards you (non-verbal cues), as well as how he says things (what is said doesn't change, its how it is stated).
For example, if you are a couple that says "I love you," watch how he says it and watch his mannerisms and body language. There will be a tendency for the body to tighten up (a form of resistance) and for the the eyes to dart or look up to the right.
Anyway, check those things first! I will follow your question.