In poker, you should always be putting a man on a hand. The best clues out there are how much he bet and when. This often narrows down the number of hands to just a few. For instance, a big raise pre-flop that checks to a board of AK2 tells me he had a pair, probably QQ JJ or TT. If the turn brings a J and he calls a medium bet, then the river brings a T and he makes a big raise, I am certain he has QQ, and a straight.
However, we usually can't narrow it to one hand. Often, there is a group of 3 or so possible hands he is holding. This is where, for me, tells come it. I need one more piece of information to break the tie between the 33.3% probability I have placed on each hand. Sometimes, tells do this for me.
So, as JG was saying, you have to have the solid foundation for starters (to narrow the range of hands) but tells may give you a bit of an extra edge on top of that.
Random thoughts from a lawyer, an accountant, a commodities trader, an ex-Marine and a WSOP Main Event money finisher that don't know as much as they wish they did...