The most common route of sabotage is messing with the crisis contribution cards. But each card had the location it was found, so all you have to do is look to see where the wrong card came from and see which character is in that location. In our 3 player games, our characters are usually all spread out in the different locations without any overlap, so it's clearly one person that contributed the wrong card. Anyone else find this to be true in their games?
If you are the betrayer and the only person that went to location X, never ever EVER spike a crisis with a card from that location. That's just bad strategy. The game gave you five starting items for a reasons, and there's a reason they are not helpful in most (any?) betrayer agendas.
But, I think avoiding spiking crises as the betrayer is better. There's enough ways to drop morale without it.