You won't know what to look for until you lap one! You can't see the problem. What happens and I don't know why but when the rings are made they aren't perfectly round on the inside. A lapping rod goes in place of the scope, you put some grit on the rod and procede to push, turn it inside the rings. At first it is tough to get going, but the more you "stroke" it the smoother it gets. After a few minutes and a little more tightening on the rings they are good to go. After you clean the grit out of the rings you will see why EVERY (except sig. zee rings) set of rings needs to be lapped. They are now truly round, giving the rings more bite against the scope tube. A out of round set of rings can actually dent a scope tube. At first I was a bit skeptical but I have now lapped 4 sets of rings from Warnes to Weavers and all were out of round.
A person can make their own lapper kit but I say it's just as easy to buy the kit, plus the kit comes with a torque driver & bits, 1" rod, 30mm rod, cd instructions and a tube of lock tite.