I've replaced two (rear) bearings on my e53 over the last few years and in both cases, normal methods used for the diagnosis of a bad wheel bearing were pretty much useless.
Because they were rears, turning loads did not affect them much and the sheer mass of the wheel, rotors etc. is so high I could not detect any movement or vibrations when spinning the wheel by hand.
Fronts might be a bit easier to diagnose and the advice offered above seems sound.
In both of my failure cases, one race on the dual-race bearing had bad galling and I was only 100% certain of the fault once the bearing was removed (destroyed in the process - since a slide hammer is the method required!).
And I tell ya - when you pull one of these bad boys, you really WANT to see this exact damage - otherwise you just pulled (and destroyed!) a good bearing!
