Paul Wilson gave a talk at DEF CON 20[1] - he said something that stuck with me (which I'm paraphrasing now):

"The way you spot a mark is to look for the person who thinks they can't be taken. They're the greenest grass to a con man."

I'm sure it's in the video somewhere :-)

[1]: https://www.youtube.com/watch?v=rkz1ItKLAvk