If I wanted to know if a mechanic had at least opened up the end of the hub to look at the bearings on a trailer that had been sitting for a while, I would look at the 'dust caps' on the hub in the center. To get the hub apart, one has to insert something like a chisel or screwdriver between the cover edge and the hub, and that should leave some marks, esp in something dirty and greasy.
Of course, the alternative way to remove the caps is to bash them sideways with a hammer, which will either leave dents in the covers or they will be new.
If you have brakes
, you can't see the inside end of the hub, but if you don't, you should be able to see that the seals are new (or old and dirty).