Sometimes it can be a hard call. When someone has given you cause to mistrust them, it's hard to ever do that again.
We can be assured that every person will bear fruit of whatever they have embraced and it will ripen and show itself. So, we have something we can count on happening.
We've, more than likely, dealt with many issues involving people coming and going. Basically, we come to the place that we put that trust in the work of the Holy Ghost. He knows the heart and he won't allow anything to destroy His people or His church. All things will be revealed.
So, we can only do as the scriptures admonish us - "Be ye wise as serpents and gentle as doves." Heavy on the wise.