I went out and played poker last week at one of the casinos here in Kansas City.
It was a generally tight (i.e. most of the players only bet on stronger hands) table but there were a few loose players who would play a much wider variety of cards. I actually knew that this table would be a tight table before I even sat down. Reason? There were 6 players who were at least 60 years of age. Generally speaking, a player over 60 is much more likely to be a conservative player. Conversely, while a player under 30 is more likely to be a loose player, you have to be careful to watch because there are some younger players that will play tightly to throw you off.
As we started to play, I noticed a common tell on a couple of the older players that you should definitely watch for in a poker room. If a player made a bet, two of the players would pick up their cards slightly intending to fold when it was their turn. This is very helpful for you as a player for two reasons. One, you can get a better idea as a player how many people will still be in the game if you already know that two of the players are folding before it is even their turn. Also, if you don't see either of those guys pick up their cards after a bet, you know there's a good chance they will be calling or even raising when it is their turn. Given that these players are probably playing pretty strong cards to begin with, you want to be very wary of their plays if they remain in the hand.
Some poker tells are involuntary, much like reflexes in a person. These are some of the best tells to detect as a player. Reason? The player can't control these reactions unless they are aware of their occurance. One common involuntary tell is breathing rate of the player. When a player gets excited about a hand or is trying to pull off a big bluff, his or her heart rate will increase dramatically. Even a poker pro's heart rate which runs 60-70 beats per minute will increase to 160-200 beats per minute during a crucial hand. Since the heart rate increases, the body will naturally increase the person's respiration rate to compensate for the increase in the heart beat. If the player is not paying attention, which they often are not, you can easily notice the increase in their breathing rate, allowing you to make decisions. If you are not sure whether the tell is that they have a monster hand or are bluffing, it is best to error on the side of a monster hand. It is far more prevelant in that situation that they have a good hand.
I actually found an excellent tell on a player at my table based on an involuntary tell. I was watching the two players that were involved in a big pot. One player bet and the second player raised. As I looked at his face, I happened to notice something about his neck of all things. I noticed that the large artery in his neck was literally popping out of his skin! His heart rate was astounding! I watched the finish of the hand to try to figure out what this tell meant. Sure enough, this guy had hit a straight. So when I saw the artery popping on his neck, that was an easy tell that he had a monster hand. I watched this tell to verify it. I later was in a hand with him and noticed it again. I only had a pair, so I got out of the hand. It ended up that he had a flush. This was excellent verification on that tell that I had a good read on him. I saw it twice more while I was at the table. The other two times, he flipped over a full house. Amazing stuff and a great way to figure out what your opponents have when you are facing them.