how do i know if i need new bearings.
Your picture is a little fuzzy, but the bearing looks OK if the top of it as shown in the picture is not rounded. Another really good clue is to take a good look at your axle where it makes contact with the bearing. If it's worn down then not only is the bearing suspect, you will probably also need another axle.
I have often wondered,is there a size to measure on the rollers to determine the wear as there is on the T car axle for a go no go test?
Also I would think the twist test would help to.
The TT rear axle bearings, were .562 when new, up to .005 wear is tolerable, just as the cars. Excessive pitting isn't allowable. Loose cages should be peened tight or replaced if to worn.