Some people have something akin to a sixth sense, be they Christian or not. Some people have the ability to discern the spirits behind a spoken word, or action. Some Christians have the bent of Prophetic Insight, and they, depending on their level of faith, are able to evaluate the integrity of those they encounter. I test and evaluate everything, and make decisions based on the nature of Christ, which is seen through His dealings with His disciples and others, and the words He spoke, and the promptings of the Holy Spirit that indwells me. People know me to be friendly and welcoming, fair and law abiding. If their spirit doesn't mesh with mine, I'm not offended. I'm responsible for my own behaviour, not others.