How can you tell if someone is a good person?

Good or bad is really a human construct or concept. Someone decided some things are acceptable and put it in the good pile and other things in the bad pile and then proceeded to label people overall as one or the other and somehow it caught on with all humans. When the truth is, we're all capable of both good or bad.
It is up to you to decide what you accept in someone (or not) and whether the things you do not accept can be overlooked by the things you value. It will be a different measure for everyone.