Does not make a difference if single axle or tandem (or triple as in some Tom Cats).
I am using Posi-lube spindles, and like to check the hub at least every month if using the trailer. If going on a long trip--then I re grease the hub (fairly easy with the Posi-lube system).
To inspect you remove the rubber plug on the outside. If there is any water or discoloration--then change. You might consider a few pumps of grease--if the excess grease which comes out--then you are good to go. If any water, milky color, or rust, then pump out all of the old grease, and replace it.
For example we took the boat to the Keys--about 800 miles. We repacked the bearings when we left home, and checked them before we where to leave the keys--squirted some grease thru the Zirc fitting and it was good. Came on home. At home we inspected the bearings and grease--all good.
Generally we repack about every two years. I will be towing a lot this summer, and will check it often.
Just running low temps does not guarantee that there is not a problem. A bearing with some water may not heat up immediately. Visual inspection also is important. Also use the IR thermometer to check the temp of the discs or drums to be sure that one is not dragging!