Dating is a precarious business at the best of times, but if you end up with a married man without knowing it, it can be very emotionally destructive. If you know he is married and go in with eyes wide open, then that is your choice. However, not knowing and getting involved before you find out is soul destroying.
Scanning the Room
Upon entering a venue, if he scans the room and sit you in the furthest corner with comments about being more intimate, then he could be married. If someone comes over to chat and he doesn't introduce you or passes you off as a colleague, then something is wrong.
Coming back to your place but never going back to his is a concern. Also, if he can only do so on weekdays, then he could well be married. Not wanting to leave your house or go anywhere is another sign. When he does take you out, if it is always well out of town, then you need to worry. Only being able to phone you briefly late at night, and possibly whispering, is a clear sign he is in another relationship.
Family and Friends
Meeting family and friends is part of a relationship, so if he never wants to meet yours and you never meet his, he is probably married.
Look for signs of a ring indentation on his wedding finger. If he takes phone calls and has to leave the room and laughs off texts from a "friend" and then has to go, he could be married. Not giving you phone numbers at work or email addresses is another sign. Being secretive about his life, in general, is a clear sign for concern.