It's too dark to check right now but I'm pretty sure it's on some top surface of the water pump behind the pulley. It's a hole about 4mm in diameter just ahead of the pump drive shaft seal. As the seal quits, coolant seeps out and leaks out that hole as a warning that major seal failure is imminent.

The test consists of nothing more than inspecting the weep hole for coolant. Your eyes and nose will tell you if there's coolant or try a q-tip.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD