you want to look at three points
the most important is the fit of the barrel lugs to the slide stop pin. If it's done right you should see little shiny spots on both where there is contact between them. This insures that the barrel is going to be pushed up and locked into the slide in a repeatable manner.
check to see how the headspace extention fits into the slide. There are differing views on this, but the ideal fit is when the barrel will move up and down freely, but you can't see any daylight except at the corners.
Some folks fitthe long surface very tight but leave the edges loose.
the final thing is the fit of the bushing to the barrel and slide. Some of the newer guns have the bushing so tight in the slide that you have to have a wrench and a couple of buddies to turn it. That's really not necessary as long as it is just snug it will be fine. You'll also see bushings so tigth that the barrel can barely move.
In order to lock up, the barrel has to be able to tip within the bushing. some are so tight that it puts a bend on the barrel when it's locked up.
