checking bearings takes experience. basicly, while its up on a lift, have someone spin it up to around 40-50mph. off & on the throttle. and listen with a stethiscope(sp).
to physicaly check the bearings. the carrier and pinion need to come out. the bearings and/or races will be pitted.
its also possible that a poor install is causing noise. i have heard of this, but have never seen it. as my installs are always perfect.