To tell if rear bearing is OK, remove half shaft bolts from hub, remove caliper and of course wheel is off. Spin the rotor. If there's any sound, time for new bearings. Grab rotor like Chuck said, determine if there's any looseness, if so, replace bearing.
Unless the bearings have been replaced already, it's a good thing to do. At least every 30 years or so. If the bearing spins in the races, you'll have a problem with the upright. It could even make matters far worse if the bearing seized while going fast. Bearings are about $150 for a set for the entire rear axle.
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