Sometimes you can really tell and other times just a few nice questions will give away what you are dealing with. I have seen some of the outrageous questions being posed in OOC. After a short conversation you'll know if they are full of it or not.
The other thing that has gotten my attention is the two box scammers in POK. One mid 20's character and like a level 10 or 11 asking for plat for a rez in the Guild hall. The last time the guys was asking me for 200 plat and funny enough for a Level 51 is only like 130 or so. Nice try.... go find a new sucker and wait for the GM to come and hunt you down.