Pre-Purchase advice

I'm interested in buying a used 1990's C class or E class. What are some things I should look for in general? I especially want to know what are some areas to look at on the cars to see if it's been in an accident - telltale signs that the average joe like me wouldn't spot unless I had been told. Also, how accurate is CARFAX - I've never used it before. A friend told me to look out for mileage rollback, that carfax wouldn't necessarily find discrepencies, etc. I was also warned there are folks out there with some kind of "equipment" that you could plug into these cars and rollback the miles. I don't want to get BURNED! Thanks for any input.

Martin in Calif.