I had UFP gold hubs on my last trailer. They had an 'inspection window' but it did darken over time and you really couldn't see anything. I have taken these completely apart after a tow through salty streets led to grit in the cup/sealing surface. They are easy to break down (you'll need snap-ring pliers), and I ran the components through the dishwasher. When reassembled with fresh fluid, the oil level in the inspection window was visible again.
I've got UFP Vault hubs on my current trailer and they do not have an inspection window....it's darkened intentionally as they claim these are maintenance free. I still think an inspection window would add a feeling of confidence, but I'm sure the evidence of a leak would be splattered all over the boat/wheels/etc.