Without a doubt the most common sign of someone stealing preflop is when they are first to act from middle to late position or in the small blind and make a raise in an attempt to pick up the blinds and antes. Recognising this opens up a wealth of re-stealing opportunities when you’re sitting in the blinds. However, just because someone has raised first to act it does not mean they are stealing, nor does it mean a re-steal is always appropriate. You should always weigh up the situation carefully before pulling the trigger.
The most important factor to consider is the likelihood that the player in question is stealing. If the initial raiser is a guy who has played three hands in the last hour, it's best to save that move for a later time. If the initial raiser is someone who has played three hands in the last orbit, however, it might be the time to make a re-steal, because he probably hasn’t got the most stringent starting hand requirements. The question then becomes how much to re-raise in order to steal the pot. This is a tough one, because your opponent has one advantage that will factor into his decision regarding whether to call, re-raise, or fold: he has position. You thus need to judge how large a raise will compel him to fold. If you make a small raise, he could very well call you to take advantage of his position post-flop.
On the other hand, you don't want to raise so much that if he moves all-in or calls you will be crippled. Ideally, you should make a raise of about two to three times your opponent’s total bet.
For example, let's say the blinds are 100/200 and you and the button each have about 7,000 in chips. The button raises to 600. You should re-raise to somewhere around 1,800-2,400 total or a raise of 1,200-1,800. This is a big enough raise to discourage an easy call but it's also small enough to give you some options should he choose to play with you.
Don’t re-steal from shorties
Another factor that players often ignore is the stack size of the initial raiser. If your opponent only has 8-15 big blinds and has opened the action, he is much less likely to be stealing than someone with a bigger stack. He might also be getting the correct price to call if you re-steal.
For example, let’s say a player has ten big blinds and opens the action for three big blinds. Yes, he probably should just open-shove but there are a surprising number of players who will do just this. If you raise enough to set him all-in there will typically be about 15 big blinds in the pot and he will have to call for seven big blinds, giving him over 2-to-1 on his money. He’d have to have a really bad hand to justify folding in this spot. Adjust your re-steals accordingly against shorter stacks.