Newer Stern's, I think starting with AC/DC, have a lifetime earnings audit that doesn't get reset with a software update. Lifetime plays may be in there too. What I'm not sure of is whether those audits reset when the batteries are pulled. If so, at least it's a little harder to completely reset audits now.
I've bought more than a couple of older Stern's that were on original code that never had their audits reset. Not everyone updates and even fewer know how to reset audits. Condition is always king, but if the game has the same software the game shipped with (check the readme file if you're not sure) and the condition matches the audits, the audits are probably accurate.