As Robert mentioned above, challenging opponents is a key part of the game, and not only to stop them from lying. We often challenge opponents on valuable roles in order to "flush" that role away from them.
I'll often make sure I have relatively low Intel just for this reason, so I can challenge others without losing much. I want to stay as close to 2 or 10 Intel in this game. A natural cycle of high and low intel in various players in the game develops nicely once you get into the strategy.
Now the real fun is convincing another player with only 2 Intel to challenge the guy with the Allies claim so you don't risk the Intel.