There are really only 2 or 3 things to check for: bearings, seals, and fin damage. Checking the bearings is the same thing as shaft play. Don't forget to check the shaft thrust, which I've been told is a sign of overboosting. Fin damage is just something you can look at. Look at how sharp the leading edges are. This will tell you if the car was run without an air filter (or a bad one). Of course, missing fins are bad as well. You really can't check seals. It is possible to have turbo with normal shaft play and bad seals. This comes from running oil too long, or again, running no air filter (sand gets into oil...).

Aside from all of that, look at the motor and see what kindof condition it's in. If it's automatic, it probably wasn't raced (as much).