It will hum... it will start quietly and wont' have much to do with RPM but
with speed. 
You'll REALLY know its going if you turn hard from side to side. If its your
front right bearing when you do a hard left turn it will hum much louder
than before, and ease off as you shift the body weight off the bearing and
onto the other side... so on and so forth. If its your rear, it will be a
little tougher to detect, but those go alot slower than the fronts
generally. 

The best thing to do is find a long hill with nobody around. Give it some
gas to get some speed up (60+km/h if you can), take your foot off the gas
and clutch in (to reduce all engine noise) and start leaning the car from
left to right. You'll hear it if its a bearing on its way out. 

Keep in mind, if its a front bearing, and you have a VR then you'll have a
day to 2 at MOST to get it replaced. Once the fronts start to go, they go
fast. 

Good luck. 
Tyler

I just had to change mine recently. The car will sound like a tank when
driving. The best way to check it is to jack the side of the car up that
you think the sound is coming from and try to move the tire/wheel back and
forth. Any lateral movement will most likely indicate a bad wheel bearing.

Sean