It depends on the situation. If no one is harmed by whatever happened and it's the first time or deviates from the person's normal habits, I will give them the benefit of the doubt until they prove otherwise. While you can't blindly trust everyone, I at least try to approach everyone I meet as if they are a kind, considerate, and respectful person.
Still sometimes I get really bad vibes from people. And then I brushed them aside, which resulted in putting myself in physical danger. A few weeks back I was possibly almost attacked by a man who was giving me weird vibes, which I just shrugged off and continued to pass on the sidewalk. He made threatening gestures and noises at me, so I ran across the street to the other half of the sidewalk and then in the opposite direction.
Sometimes it's just better to act on your gut instinct, which I should have done from the get go there. When I was about 14, a friend and I walked to get ice cream. A man was sitting outside a car outside of the shop. And he asked us if we wanted a free potato. It seems laughable in hindsight, but we were freaked out by this guy and ran away. To which I wonder, who tries to lure young girls in with vegetables? But, it's often better safe than sorry.