Just have a look at the general condition of the AVP. Look at the cussion of the light seal whether it is sweaty or greasy. Also have a look at the front glass. There were some AVP that developed a vertical crack right in the middle above the nose starting at the bottom. (Apple repeiars it for free, but it is a flaw)
Also look at the inside at the "screens" - scratches from cleaning.
Running it - it runs or it does not run. Nothing special to control.