Inspection is real simple. Lift the tire (I use a rapid jack on the other tire) and spin it listening and feeling for smooth operation. Grab the outside of the tire and push back and forth to check axial play. There should be the tiniest bit of play. After a few years I can tell what the play is as it is consistent among all four wheels. Your only option is to replace them if you feel uncomfortable with their lifespan.
Your trailer sounds heavy and if original they are ten years old. Mine are also 10 years old but on a lighter trailer. I plan to run mine until inspection detects a problem or have a roadside fail. I carry an extra drum with bearing pressed in plus the tools needed to replace in an emergency. When the time comes I will buy a 20 ton press from HF and switch out new bearings myself.
Dexter quantifies the exact amount of axial play on their website.