This is a topic that will be of interest forever, and it will always be evolving as people discover more and more tells … not to mention the fact that as players get better and better, they start faking them … so far, i have discovered the following tells from personal experience.
Strength
1- If someone starts making odd bets ie non- round numbers eg 1,475 or 2,150 … they are definitely strong … they have something very close to the nuts and will call an all-in bet … so go ahead and put them all-in if you have the nuts … otherwise fold.
2. After making a bet, if someone starts looking around for their drink or the waitress or simply is not paying attention to what you are doing, they definitely have a strong hand and will call an all-in bet … conversely, if they start doing that before they bet or before you have bet, they are about to fold … it just shows that they have already made up their mind … obviously if they bet and have made up their mind, they are strong.
3. When people start breathing heavy, it's because their heart is beating fast which generally means that they have a strong hand … you'll know that when you see it, you'll see their chest rise significantly at a fairly high pace … same as shaky hands.
4. If someone keeps checking their hole cards, they have a monster.
5- Minimum raise ie twice the blind or twice the amount you bet … is almost always a tell of strength.
6. Going absolutely quiet is a tell of strength for most people … but beware, it is the easiest to fake also, so you need to know your opponent.
7. If someone slides their chips in very quietly as if you almost did not notice that they raised, they are strong … but probably not too strong … they have a good hand but they are afraid of something.
8. If someone is eating food and they play a hand, they are generally strong.
Weakness
1- Someone starts talking too much … they are generally weak … accept for pros like Jamie Gold or Negraenu, who can talk at anytime.
2. Someone puts in their chips forcefully, that means they have something decent, but definitely not the nuts or close to that … for example, if there is a straight and / or flush on the board, they do not have it … they might have a set or a two-pair.
3. Someone splashes the pot, they are generally not that strong … but beware, that person is probably on tilt and will call an all-in bet out of spite.
4. When someone keeps staring at the flop or somewhere on the table but is not willing to look at you, they are afraid of something … they might have a medium pocket-pair with one or two over cards on the board.
5- When someone tries to stare you down … they are not that strong … a good idea is to ask them how many chips they have left … that will generally make them blush or their shoulders will drop a little which is definitely a tell of weakness … that does not mean that they are ready to fold just yet, they might call one bet … but they will probably not call the second bet … ie they bet the flop ( and stare at you), you raise (after asking them to count their chips) … they'll probably call … they'll check the turn, if you bet a medium amount (say 30-40% of the pot ) they'll probably fold.