[kitchencounsel]

There's no way to know, of course. I don't worry about round counts; I look at condition. If a gun has seen use but has been well maintained, that's probably a go.
If a gun shows little wear but shows signs of neglect, I'll pass.

Take it home, then shoot it and see if it needs anything. Springs are cheap. Barrels are available at many price levels.

Just like with a used car, you need to allow some money to fix whatever it was that caused the seller to sell.
Most of the time with guns, that turns out not to be necessary. But it might be, so factor that in.

For example, not too long ago I decided to get a "beater" 1911 to keep in my truck. I picked up a Thompson Ordnance 1911A1, for no other reason that I liked the look of the bullet logo with the cursive script on a blued gun.

I'd heard all the horror stories about those guns being crap or whatever, but research seemed to indicate that the problem was likely to be the result of early MIM parts or maybe GI-reject parts.

So I looked at the cost of new internals (fire control), new recoil and/or main spring, and maybe a new barrel and mentally added that to the price of the gun.
So I found a cheap one online that looked to be in good condition with a nice finish, and appeared to have seen some use (which meant it had been working well enough for someone; a good sign), crossed my fingers and bought it.

It shot fine; though it wasn't ejecting too well a new recoil spring fixed that.
A little polishing of the internals, fixing the sear profile and hammer spurs, etc. and it's greatly improved.
I dabbed a little Oxpho Blue here and there, and it looks almost new.

Since I didn't have to really do anything remedial, I blew the contingency money on a Wilson extended slide release and thumb safety, replaced that crappy-looking trigger and the flat MSH for an arched one, and decided to replace the hammer spring for good measure. The original grips were worn, so I dropped ten bucks on a pair of GI contract plastic grip panels.

Now it's too nice to keep in the truck!