You can either hold the axle and spin the wheel up and feel through your fingers for any rumbling or tight spots trying to turn the axle in your fingers, or take the chain off and tighten the wheel back in the frame and spin the wheel up and feel or listen on the chain stay with a screwdriver [stethoscope stylee] for the same
As Blickblocks said the cones may be too tight if the lock nut comes loose they can cause the inner cone to tighten up on the axle bearings and cause seizure.
If you want to do it properly strip the axle out and clean off the grease and look for any pitted marks in the case hardenened surface of the bearings surfaces.
If they are sealed bearings well just change them, they are usually cheap enough, replacements for Goldtechs are £12 a pair
Failing all that if you're still not sure take it to your local bike shop