The voltage spec in the FSM (15-5010) says 13V-14.5V @ 3000RPM.
If it's above 14.5V
1. Replace regulator
2. Replace alternator if regulator is in order.
Below 13V
1. Rectify contact resistance at electrical connections of charging system
2. Perform diode test (in a section below).
I'm not sure you can do the diode tests with a multimeter (some tests for harmonics - determines whether to replace the regulator or alternator - if the harmonics are fine, you replace the regulator, if not, replace the alternator). There are scope pictures detailing what each case is in the FSM.
http://www.ps2cho.net/downloads/MB%20CD/W124/Index/602_603/15EngineElec.htm