sometimes if you tell someone about something that makes you vulnerable--ie a fear, insecurity or something in the past that hurts you, if they use that information or recreate that situation against you, it means they're manipulative and can't be trusted.
sometimes i say things and remember them, so if a person comes back to use what i'd "revealed" to him against me or if he tries to lie, i know he can't be trusted. once i told a guy that when people start yelling, i can get freaked out and can't remembered what had been said for the argument to have gotten to an escalated place. my point was that i prefer that things be resolved calmly and in the spirit of people working towards a common goal. a few weeks later, he picked an argument with me and then claimed that he was right because i'd forgotten what we'd said that led to that argument, and lied with a line that i had supposedly said. unfortunately for him, i don't forget dialogue and conversation flow even though i'd implied that i do, and knew he'd just lied during a moment of perceived weakness on my part to dominate me. he'd stepped on a wire that set off a trap, blowing his chances of my ability to trust him. he showed his true colors after i protected myself and guarded myself from him, and now i'm sure he's off harassing other girls until they see the proof of his manipulative streak and his lies.
especially in this city which is dominated by an industry whose lifeblood is about being able to disguise oneself or be something that you're not, it's really important to be open to trusting good people, but not taking everyone at face value.
i don't believe in consciously booby trapping the inner workings of close relationships because people trip and make mistakes on the way to building partnerships and getting to know people, and this is a necessary part of the process. but i do try to have ways in which i can figure out sooner rather than later if a person is dishonest, manipulative or has negative ulterior motives. it's better to keep untrustworthy people outside of the gates, than kick them out once they're inside them.