I try to emulate the views of an impartial person in internal monologue. I think it helps to engage in a lot of debates and discussions and take pains to observe the differences between people who have incentives to engage in motivated reasoning and people who don't, so you can notice in yourself "That doesn't seem like a way I'd respond if I weren't engaging in motivated reasoning," or "that really does seem like what I'd expect from an unbiased person."