Actually there's really good primers here: http://www.etrailer.com/tv-repack_tr..._bearings.aspx
My best advice is this:
1. Temperature. Touch your hubs after running for 50 miles and get a feel for how warm they get. Very hot hubs may mean you are running dry and need to grease up or theres a mechanical failure. Could also be heat generated from braking so you have to develop a "touch" or feel for this test.
2. Visual. the wrong grease, mechanical failure, or incorrect reassembly will allow grease to sling out of the hub and get the wheels greasy.
3. Inspection. Put the trailer on a jack and grab the mounted mounted wheel like a steering wheel and check for play. It should have a small amount of wiggle or the hub is installed too tight. If you don't have an oil bath or grease buddy caps, pull off the dust cap and see how the grease looks. If there is a cavity where there should be grease, inspect the bearings before a very long trip occurs. If the grease is settled at the bottom, you parked the trailer with hot hubs. That's no problem. Just remember to start slow next trip to redistribute the grease. When I say inpect the bearings - that is really a bearing job because grease seals cannot be reused. I have reused grease seals to cut corners and lost the savings when I had to constantly clean up grease leaks and regrease the hubs.
4. Run the trailer LEVEL. That assures weight is even on all wheels. This reduces bearing wear, sway, and uneven tire wear.
For the first time, I would get a tech to do it and ask to save the parts. Look at them to get used to how they wear and score. Get another replacement set for all axles matching them to the old parts for the next job. Easy stuff.