For those folks with the E-Z-Lube hubs with the charge nipple in the end of the axle stub; you can grease those reasonably safely without pushing lube out past the rear seal by waiting until you've come to a rest stop after an interstate haul. Just make sure it's not a cold day and your grease gun isn't kept in a cabinet frozen solid.
The trick is to make sure hub, and grease are as warm as you can get them and add grease with a slow stroke of the handle with only a couple of strokes being required.
The best way of course is to disassemble and inspect because everything from brake shoes to magnets should be checked seasonally in any case, then repacking by hand with a good amount of grease packed into the hub cavity as well. You want enough in the cavity that if enough heat is generated that liquification occurs, it runs TOWARDS the bearing and not away from it.
Last but not least make an effort to replace the rear seals yearly and with a double lip spring supported seal of better quality than either standard OEM or those included in some rebuild "kits."
Chicago Rawhide or CR have any number of conversion double lip seals in all sizes to suit just remember the WIDTH of those replacement seals is every bit as critical as the I/D & O/D. I have yet to see one though that would have been too wide to seat flush with the rear of the housing.