I usually believe people when they tell me something, unless there are things about it that doesn't make sense that makes me suspicious. Like if someone told me that they were at the store, and a friend tells me they saw them at the mall a few minutes beforehand. But I'm usually incredibly indifferent about things people tell me, so I think that's a contribution to my trusting them. Example:
"Hey, I just found $400.00 on the street!"
"Okay...."
So most of the time it's just like, okay? Why are you telling me this?
For peer pressure, I don't really give into that, I do the opposite because I like being difficult.