The symptoms are usually a rumble, heard particularly when turning one way or the other, but continuing with the engine ticking over and out of gear (that is, foot on clutch, foot off accellerator). It sounds like driving over fine ridges in the road.
Replacement depends upon the car. Usually the parts are not expensive, and sometimes it's just a matter of dismantling the hubs, replacing everything and adjusting correctly. Personally I've never needed a press, though as Andy says some bearings do need one.
I've found them to be relatively cheap when done by a garage. Sometimes they just need adjustment or packing with grease.