Look for sweat on the brow of the guy whose selling it.
Look for stripped screws, glue on bottom of knobs, cracked LCD, loose side panels (dropped), or other signs of abuse.
Trust your people skills and your instincts. Take your time with it and try it out. It's a lot of cash!
The best case would probably be going to the house/studio of the person you are buying it from and seeing alot of other good quality, well maintained gear...
