Looks for marks where the metal rubs on metal. Look for anything unusual. Example is on ARs, look for parts where there's anodizing, but parts end up exposing silver. Not scientific, but unusual wear can be indicator of a problem. That said, how would you know what is usual or not? Best I can offer is research online or talk to experienced folks if you notice problems.
It's difficult to tell sometimes if wear is due to normal "break-in" friction, or if it's excessive. Adding more lube isn't always a good solution. Most pistols don't need more than a couple of drops in the places where metal contacts metal, For a Glock, that's not much at all. In fact, if the pistol is overly oiled, it's going to pick up dirt and dust, cause stains on almost everything it touches, and can become slippery.
When it comes to lubricating pistols, less is more. That's why I like FrogLube. It coats the gun completely yet feels dry to the touch. But, when you use it, the heat melts the FrogLube and lubricates the internal parts.
I'd suggest if you see parts wearing through the finish or showing signs of scraping, gouging or uneven wear, call the manufacturer. See if they will let you send pics of the suspicious areas for analysis.