Inspecting a used semi auto is a little more complicated than most people realize. You can give it a good look over at a gun shop but you really have to take it apart to give it a good inspection.
I've never been in a gun shop that would let you take apart a used gun to look at before you buy it. The first thing you look at is the finish, that will give you an idea on how much the gun has been used. There's nothing the matter with a gun having a lot of honest holster wear but the price should reflect this. Some guns like police trade in's have a lot of surface wear but usually have not been used a lot.
You have to be a little wary about guns with new parts or obvious modifications, there are a lot of amateur gun smiths and while that's not a bad thing if it's just new sights, they may have gotten the pistol assembled wrong but working by doing some thing like using the wrong pins or filing down the firing pin stop to much.
You have to check for cracks on the frame and slide. The most common place for a crack to develop is where the slide stop go's in through the frame.
You want to check and make sure the gun and barrel are reasonable clean and show no signs of rust, pitting or peening and the barrel still has rifling and is clean with no leading or pitting Take a small flash light with you and lock back the slide check the barrel and while you have the frame locked back check the rest of the pistol and make sure there's no rust and it's clean.
Then you want to work the slide and all the controls to make sure their working smoothly and not binding or stuck. Look at the barrel in the slide and see if it has a lot of rubbing marks that's not a good sign. Some semi autos do show some slight rubbing marks but a 1911 shouldn't.
The only semi that should NOT be dry fired is a 22, all others can be safely dry fired. So ask first to dry fire the gun, if they say no you have to wonder why.
If they say go a head, point it in a safe direction and keep your finger off the trigger and cock the hammer and then press your thumb against the hammer to make sure it won't move. If the hammer falls you know there's some thing wrong and it needs work also it's unsafe. If the hammer stays put then put your finger on the trigger and dry firing it.
I've passed on some pistols after every thing seemed fine just due to the way the trigger felt. A gun with a super light trigger pull like 3 pounds might be all right for the range but you wouldn't want to really carry it. A gun with a real heavy trigger will need a trigger job.
If you buy it take it apart right away and inspect it.