It’s simple: don’t trust anyone unless you have known them for a full blown year. People who are pretending or lying to you, will not be able to sustain a lie for so long. If the person lasts the whole year, and has given you good proofs that he or she is a good trustworthy person, then that person is a keeper.
Don’t stress it. Like my Dad always says, “Don’t be too trusting, but neither too un-trusting.”