What do you actually check on a pre-trip inspection?

They're opaque because it's grease packed but those are the "sight windows". They're clear on oil bath and have "low - high" oil level marks.

 

This is an oil bath hub cover. Looks very similar to the one you showed but slightly different. that's clear plastic in the middle so you can see the oil level of the hub

 

It’s intended as an oil bath cover. But it’s either so old or someone’s packed grease in there that you can’t see through the windows.

 

Looks awfully dirty to me, wipe it off and look again.
We have some old trailers that you can't see the markings on if they are dirty like that and some are so bad you actually need to shine a light into them to see the level or markings.

 

Radio, air, heat.