1. Being asked to send in a deposit first for a local provider.  (Deposits are common if you are flying in a person though.)

2. A provider who discusses money and/or specific sex acts on the phone or email (possible LE sting)

3. A provider who does not screen.  (LE sting also).

4.  Sound too good to be true.  (Photos of a fashion model and FS for $100/hr, for example)

5.  Meeting in a shady or isolated place like an alley or deserted road.  (You are probably going to get mugged.)

6.  Someone who requests you SSN.  (Identity theft coming)

7.  Information about your bank account number.  (bye-bye bank account)

8.  Craig's List (Not everyone on it is a scam artist or LE, but from what I read here it seems like a good way to meet those guys.)

9.  AMPS  (Asian Massage Parlors)  (At one time, not a bad way to meet some nice providers and get a good massage also, but with the heat on the immigration issue, they have been getting busted quite a bit over the last year.  If you do decided to try one out, be carefull.)

10.  The streets.  (What more do I need to say?)

There's a quick ten without even trying hard.

The moral of the story:

Click on the TER Instruction Manual above and read, read, read.

Thier is no sure sign, but watch for some things others have mentioned.  But mostly follow your gut feeling.  If something seems to good to be true or something just does not seem right, break it off and walk away.  
I've been scamed a few times and every time when I looked back something seemed a little odd about the way things were going.
I have also walked away and cancelled because I thought something was not right.  
Don't let the little head overrule your big one, and more often then not you will be fine.