Reading the comments here gave me an idea....google your own credit card numbers to check if its already in some scammer's index. While no results might not necessarily mean you are safe, a positive match is a clear red flag.
credit card, or if paranoid, partial credit card # "123456789"
Email address
Home address
Phone number
screen name (e.g.: "andrenotgiant" -site:ycombinator.com)
Bank acct partial
Password, Password partial "p@ssw0r"