Disconnect the belt and spin the driven motor. If it sounds rough, seems tight, the fan freewheels for a long time, or the bearings make a clicking/clanking sound as it spins, plan on replacing the bearings at a minimum. Good bearings will be smooth and quiet. They'll also have a very slight drag to them due to the grease inside.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]