i spoke to a collector who was as close to a being a reel expert as anyone I have ever met. Really a center pin isnt much more than a backing plate, spindle, bearing/bushing, and drum so I think they are relatively easy to inspect.
Obviously if the spool spins freely then the main part of the reel is fine...just give it a twirl while holding the reel in your hand. one thing you can watch is the level between the backing plate and the edge of the drum. It should be at one constant level indicating the reel is still running true. While the drum is spinning, try changing the angle of the reel in your hand, shift it bac and forth. it should feel "tight" if that makes any sense, the spool shouldnt shift at all.
When the spool isnt moving, gently lift and depress the drum up and down on the spindle. this will show how much play is allowed for the drum to more along the spindle. There should be very little movement here. On an avon the is a small screew to make adjustments but I have found that turning that screw can effect the free spool action.
One general comment. Buy a reel that is totally exposed with no line guard. If you get a backlash then being able to pull the line off the spool helps. I have one old spoked center pin and opposed to a backing plate, its more of a cage. The line runs between one of 4 posts and that set up can be a drag if you have an overrun.
good luck