knowing a good and bad challange

When a tough call is made by the referee and the coach disagrees with the ref, the coach has the right to challenge the play.

To decide a good and bad challenge watch the replay and look how it follows with the rules. The odds are that about 70% of all challenges have the ruling on the field standing and 30% have the play turned around. Also see the coaches reaction for the challenge. If he goes insane, he is most likely seeing something that the ref did not see. And if he seems calm its most likely that its a tough call and he cant make up his mind