Depends on your goals what you should look for.
Here's about all the bases covered:
http://7.62x54r.net/
Look down the page under Collectors/Rarity if you are looking to get more than your $100 worth for your $100 rifle as most retailers get large lots and have no idea what they've received.
If you have visions of having a rifle routinely capable of under 3MOA, look for one with a Boxed SA stamped on the barrel shank.