You can test the ALDA with a MityVac that can produce pressure, like the Silverline Plus 4050 kit from thetoowarehouse.net (for ~$60 I think). It should hold pressure and bleed down slowly below 10psi, and hold pressure steady above 10psi. If it bleeds down fast (watch the needle swoop down) or won't hold any pressure, the shaft seal is bad. It's not hard to replace but good luck finding a Bosch shop who can identify and/or order the seal.
If the internal diaphragms rupture, they won't press down on the I/P shaft, which will cause a very rich condition with no boost (black smoke, low power). This is harder to test for. You need to remove the ALDA, press gently on the output shaft in the hole (with a small screwdriver or something), then apply pressure to the input port with the MityVac. The shaft should move upwards (into the ALDA body) as pressure increases. It's kind of hard to explain.
Photos of the ALDA internals are here:
http://www.meimann.com/images/mercedes/OM603_intake/
Regards,