How do you know if a potential creeper is really a creeper?

On the U-bahn tonight I let a guy use my cell phone.  He was wearing kind of dirty, baggy, black clothes.  He had a desperate look on his face.  He spoke in a mixture of German and English. I won't deny that at first I tried to pretend I was a dumb tourist who only spoke English (everyone does this in Vienna if they get in a bind where being stupid will save them), but after he started speaking English, too, I couldn't really rely on that any more.  At first he walked away, but then he came back to me.  I know I have an innocent face, but I am slightly amazed at how much of a push over I must look like if he was willing to walk to the other end of the train and then come back to me just for a second try.  Obviously I let him use my phone.  I decided that even if he made a call to some foreign land, the most I could lose was the 9 Euro I knew I had on the phone.  I was willing to bargain with 9 Euro in order to help a bro out.  I was also surprised, though, by how hard it was for me to decide what to do.  The little people on my shoulders--I don't consider this a moral question of right versus wrong, so I am not using an angel and a devil as the little people on my shoulder. Actually I'm going to use power rangers.  The blue ranger on my right was saying of course you should let him use your phone!  He needs help, and it isn't a huge sacrifice for you to let him use it for a few minutes.  But then the red ranger, Jason, was like but wait, Katy!  You don't even know who this guy is!  He could run away with your phone or call his mother in Zimbabwe or just not give it back!  Then he could stalk you and follow you to Waltraud's house, and it would be an Austrian nightmare! I listened to the blue ranger because Billy is the nicest one and also the smartest, and I think sometimes you just have to be brave and take the plunge even when you aren't sure what is going to happen.  The moral of the story is:

GIVE DUE CONSIDERATION TO THE RED RANGER, 

BUT ALWAYS GO WITH THE BLUE RANGER IN THE END.

Thank you and goodnight.