How do they act when they burn up ? Hot and fire ? or just get loose ? I've been way too lucky when it comes to trailer bearing failure , I'm sure my day is coming.
Prior to pulling the trailer you could jack it up, wiggle each wheel and see if any are loose. Loose wheel = new bearing or new hub, whichever you're inclined to do. If you've pulled a ways and want to check then put your hand close to the hub (DO NOT JUST GRAB IT) and feel for heat. You could also make a couple of corners and watch the inside wheels in your side mirror to see if any of them are "leaning". When it's getting close to "going" you should hear a metal on metal squealing and you should stop as quickly as possible and deal with the situation because you're about to stop no matter what.
The trailer under my '06 1850 Crestliner FishHawk got a brand new axle under it last yr because I thought the squealing I heard when filling up @ fleet farm was just the trailer hitch/ball doing it's thing. I made it from Fargo to Devils but only made it half-way home and had to leave my boat on an approach and then deal with a towing company and the new axle business all over the phone. It would've been less than a 1hr job to walk into Fleet Farm, grab a new hub, jack up and remove the rim, pry off the Bearning Buddy, remove the cotter pin and castle nut, slip the old hub off, slip the new one on, re-install the castle nut/cotter pin, rubber mallet the BB back in place, and re-install the rim. Instead it was a $300 tow and 10 days for the new axle to arrive and my turn to come up in the repair shop's order of business which was another ~$1,100 for parts and labor.
New axle vs. new spindle: Everyone needs to make that decision for themselves. My trailer sees 5K-7500 mi/summer, sometimes more, sometimes less but I figure that trailer had 50-60Kmi on it as it's on it's 3rd set of tires. I guess I just didn't trust that anyone could cut off the old spindle, weld on a new one, and then make it straight enough to not be chewing up tires.