“Saint Anthony once wrote about having gone into the desert on silent retreat and being assaulted by all manner of visions–devils and angels, both. He said, in his solitude, he sometimes encountered devils who looked like angels, and other times he found angels who looked like devils. When asked how he could tell the difference, the saint said that you can only tell which is which by the way you feel after the creature has left your company.”
–Elizabeth Gilbert, Eat, Pray, Love pg. 189
I would like to think that this applies to people as well.