Mercedes-Benz: Auxiliary Rad Fan - 1987 300sdl

I have a 1987 300SDL which is beginning the develop a tendency to run hot.
I know this a problem for the aluminum head engine. What I need to know,
is: when should the auxiliary fan become operational (i.e. what
temperature), and how does one test whether the auxiliary fan is working
properly?

100 degrees C.

Pull the lead off the temperature sensor, start the engine and ground
the lead to the engine, aux fan should start running and stop the moment
you remove lead from the ground.

You're right to be concerned about this engine running hot. Be sure the
radiator is clean - inside and outside.