I have come to say a quick prayer when I see one approaching - "Lord, is he with you?" The reason I do is because there were a couple of times when I was being tested and I failed miserably. "Be not forgetful to entertain strangers: for thereby some have entertained angels unawares". It was especially damning because at these particular times in my life I had been praying for the Lord's help in my own life and yet, given an opportunity to prove myself worthy, I was unwilling to take a chance on helping someone else because I just projected the worst, cynical aspects of my own existence on to them. And even if they aren't angels, they were still God's children.
If you are convinced of nothing beyond a cartesian world, then none of this matters anyway, and if Jesus is your personal Savior, you will know exactly what I'm talking about. You know right away when you fail in His eyes.
Just pray for discernment, that's my advice.