The thickener in grease (think of a chemical sponge) is designed along with the oil in mind for peak operating temperatures. If the thickener fails, the oil won't be held in place and the bearing will start to leak. While this may be a good short-term signal that the grease needs replacing, I know you're asking more for a long-term signal. Short of getting a sample from inside the bearing, I can only say that many race series monitor bearing temperatures during the race (i.e. at Indy the most heavily loaded wheel bearing is the RR) and they have a pretty good handle on typical, nervous, and imminent failure temperatures. Maybe some of the stick-on strips can be applied ... but you'll have to build the database and experience a failure to know.
In industrial settings there are also vibration monitors. Again, this is after the point that re-packing the bearing is going to fix it.
Can you name a recommended grease for a rebuildable wheel bearing? I'm curious.