The closest I've been able to tell is with my Ruger M77 (slide safety) in .257 Roberts and my Marlin 1985 in .45-70.
The Ruger was made in 1989 and I acquired it in January of 2004. There were no marks in the bluing under the slide safety, something I "fixed" in the first couple trips to the range. I was told by the seller it had maybe a box of ammo through it and the count couldn't have been far off.
When I first saw the Marlin the serial number told me immediately it was made in the year 2000. I picked it up in November 2002. The Marlin was still in great condition and I figured someone got it, shot it enought to figure out it wasn't for them, and got rid of it. I picked it up at a gun shop for $249.95. The condition of the rifle, it's relative newness and the type of cartridge all combined suggested a low round count. Could be wrong but I doubt it.
Mostly I look for signs of neglect or abuse when buying used firearms. So far the "worst" used firearm I've purchased was a Ruger .357 Mag stainless Blackhawk. It looked great but the fires cases would hang up as they rotated around. Sent the pistol to Ruger, got a fix for free (new basepin) and its good as new.
The other used frearms all shoot great.