The forcing cone is the funnel-like mouth at the rear of a revolver barrel. If it looks worn down this generally means a high round count on a revolver, can't hide the revolvers age there, unless replaced, if possible and fitted with a new barrel. Likewise, I recall one of the moderators put together a thread on what to look for in a purchase of a used firearm.

Um, I believe that this question is about a semi-auto, not a revolver.

Take the gun to a qualified pistolsmith, and have it evaluated for wear and function. Whatever you have to pay for this service, it'll be worth it.

Personally I look for damage more than anything on a pistol. The only thing I really do looking for wear is checking the barrel's rifling for "sharpness". Don't know how to describe that really, other than a distinct edge on the grooves and if it's a glock, or other octagonally rifled barrel, a clear definition between the lands and groves. A safety/function check later and some bargaining and I've got me another piece to add to my collection

Any exterior wear just tells you if the gun was a prized posession or a tool.The bore and lockup areas are your best indicators of where it sits roughly in it's service life.Round count is impossible to know because alot of target ammo can show similar wear to a less amount of hotter ammo.Knowing what the gun is may get more specific answers.