Originally Posted by parktools

There is a fairly simple test to determine if the bottom bracket bearing is worn out. Shift the chain to the inner most rear sprockets front and rear. Drop the chain off the smallest front ring, and arrange it so it will not strike the chainrings. Spin the crank while holding the bike with one hand. If you feel an obvious rumbling or grinding feeling, the bearing are wore out and the unit should be replaced. Very worn bottom brackets will actually make a grinding noise.