To determine a bad evaporator:
The first clue is warm air coming out of the vents during the summer time. After that event you check to see if the refrigerant has leaked out of the system.
You then must recharge the system and put in UV dye. You watch for the UV dye with a black light. If the blacklight shows the UV dye in the condensation drain water below the car, the evaporator is leaking.
Under shop conditions, you can put in 4 oz. of R22 and pressurize with nitrogen, then use an electronic sniffer. Since you can't get to the evaporator very easily, it's not the best way.
This is NOT what you want to happen to your 124 MB. I cross my fingers every spring, and so far I've been lucky.