Unless they've changed the format somewhat, then the players never officially get to "guess" who the mole is, they just do the quiz at the end of each episode. In the quiz, they answer questions about the identity of the mole (the quiz can be seen at
The Mole - Preview the Quiz - ABC.com ). I don't think they're ever told their score on the quiz, just that they didn't get eliminated, thus, they would never know how well they did on their guesses, or how close they were in guessing the identity, only that they didn't come in last.
Strategy-wise, when taking the quiz, assuming one could even remember every answer about a person they suspected was the mole (that'd be hard in itself, imo), I think it would be bad strategy to answer that way, because if you filled out all your answers based on one person and you were wrong, you'd have a good chance to scoring very low on the quiz, and possibly being eliminated.
So, basically what happens is, it comes down to three players at the end - one of them is the mole, two aren't, and the they all have to do a final quiz. The non-mole person who scores the highest on that wins. On the last episode, after the mole is revealed. they always go back over the season and show you what was done by him or her to sabotage the game, and they show you clues they've thrown out there along the way.