Tude, touching. If a guy is really into you, they will touch you. I don't mean groping or anything sexual. It might be playful. It might be a hand on a shoulder, or it might be a hand behind the back when they open the door for you. I don't mean they do it once. I mean fairly often.
I'm speaking at a higher level than just flirting. They will flirt too and you can pretty much gauge it at work, because you are talking with that person a lot. Married, single..doesn't matter. They all do it the same way. The married men are more subtle, but touch + flirting is a sign that they are into you.