Welcome to G&G Steve! A good idea is to go here:
Rarity of Mosin Nagant Rifle Variations Print the page and carry it with you, so when you stumble onto some Mosins you can quickly go through them. Works for me!
I usually use visual clues when I first look at Mosins. Any hex receivers? Brass handguards? That kind of thing. Look for different sling excutcheons as well.
Being an exsniper does not necessarily mean accuracy. There is a reason they are
EX snipers after all! They would be a good bet if you wanted to build a sniper repro though. Then start looking at markings, Tula or Izhevsk, matching serial numbers, stock cartouches etc. And of course price! Does it come with the usual accessories or not? Hope you find a great one! Don't forget to send us a pic!