-A good, non-cracked stock is the first thing I always look for.
-Personally I like to have a rifle with all matching parts, check the bolt, reciever, magazine floorplate, and buttstock for serial numbers and make sure they all match. It isn't a big deal if they don't, I just prefer it that way.
-Ask to see the bore. take the bolt out of the weapon (by opening the bolt, pulling the trigger, and (with the trigger depressed) slide the bolt out of the rear of the action.) and point it at a light source (overhead light, sun, etc.). Like any weapon a smooth, shiny bore is best without any visible pitting.
-Check the crown for damage and counterboring. It doesn't make a difference if the rifle is counterbored, its actually preferred over a damaged crown.
-Check all visible surfaces (including the bolt and inner magazine) for rust, pitting, etc.
-If you can find a rifle with [SA] stamped on the receiver, it means its a Finnish capture, and is likely in better condition and a more accurate shooter than the non-captures.
-Cycle the bolt and make sure it actually goes "click" when you pull the trigger. The rifle has likely been sitting in a warehouse for the last 50 years, so expect the action to be a little stiff.
There isn't much to it other than that. Mosin's are fairly indestructible, with the most common point of failure the stock.
Check out
http://7.62x54r.net/ for more information on those rifles than you could ever want. Enjoy spending hours cleaning the cosmoline from every nook and cranny