I borrowed a friend's trailerable mono to use as a comfy Race Comittee boat one time. While trailering it back to his house, it suddenly got wobbly. I pulled over quickly, and went back to the trailer. The bearings had siezed up, shattered, and the wheel was barely on the axle. Only reason it didn't come completely off is because it was on the right side, and I pulled off to the right.
I removed the wheel, gathered the bits & pieces of the bearing, and went to the nearest boat store, with a sad look on my face. They parts guy saw me coming with the greasy bits in my hand, and started laughing. Said he got at least one person a month come in for bearings, usually took them 3 trips to get the right size. As I described the trailer (what diameter is the axle, which bearing - inner race or outer race - huh?) he started looking at the bits & pieces I had brought in. Turns out one of them was big enough to actually contain a legible part number, which he cross-referenced, and found out a replacement. I went back out to the trailer, installed it successfully.
Now if I have to borrow a trailer I'm unfamiliar with, I usually offer to check the bearings as compensation for borrowing it. I figure it's a cheap price to pay compared to a breakdown with a full load on a highway.