Usually I think, I'm very much a 'dont judge a book by it's cover' type of person, but if someone sets out to be deceptive it depends on how good they are at it whether you catch on or not.
I can only seem to judge bad characters. A bit like knowing someone is a knob a few seconds after they open their mouths. I always give them the benefit of the doubt though.
I like to give everyone a chance, but I do trust my gut feelings about someone on first meetings. As more often or not my initial impression of someone turns out to be correct.