I think it's important to know--
IF you have a bad seal and pump grease into the zerk it
can wind up on your brakes.
There's no easy/good way to know if you have
bad seals or not.
I believe you will be less likely to "grease your brakes" if you
raise the axle and slowly turn the wheel while you grease it.
There is no guarantee here but I believe it's good practice
to what Lou suggests.
We towed our new trailer 2 years without checking
our bearings. Once I did disassemble them I found at
least some grease in each hub but the were not "full".
I think I was lucky. I also agree that most dealers will
NOT spend the time and labor to visually inspect your
brakes or bearings on a new trailer. I agree that the
axle manufacturer should completely fill the hubs with
grease before shipping them to the trailer assembler.
It doesn't happen.
You should either fully fill your new hubs with grease
or get your dealer to do this. I consider this cheap
insurance against wheel bearing failure.
Sitting on the side of the road with your wheel off
in the weeds sucks, whether you are in warranty or not.
Once I decided to check them I just went
online and bought wheel/bearing kits.
They were cheap enough and each one came with
bearings, double lip seals and new cotter pins and washers.
My axles did not have grease zerks in them but they were tapped
and ready to accept them.
I removed each wheel and brake drum.
On my trailer the brake drum and bearing hub are one piece.
I carefully inspected each brake for unusual wear and made sure
wires and coils were not rubbing and/or damaged.
I drove out the old seals and bearing races (cups) and drove in new ones.
I installed new bearings and seals and re-assembled
I slowly turned each wheel while I pumped each hub full of grease.
It took almost 2 full large cartridges to fill 4 hubs.
I adjusted brake shoes on each wheel before lowering it.
It was a 2 afternoon 4 beer operation.
We drove 4500 miles this summer and I checked the hubs at
several of my fuel stops by touching with my hand.
They all ran slightly warm which I consider normal.
Remember that some brake drum heat is normal and
if you've been driving in stop and go traffic or hills/mountains
they will be quite warm or hot to the touch.
I don't plan on doing much of anything else to the bearings
for a couple more years.
I plan to adjust my brakes and check for play in the
hubs this spring but I don't plan to open them up
I wish I could remove the drums to inspect brakes without
pulling the hubs but mine are one piece--