89 300e: How can I be sure it's my fuel pump thats bad
 

I believe my wife's 300e fuel pump is going out. I know there's two of them and one of them is humming and making alot of noise for about two weeks now. But just today the car is acting like it's not getting enough fuel when you try to accelerate.
I guess what I'm trying to find out is how do I know which fuel pump is bad or if it could be something else, and how much will I spend on one

MB put two pumps to provide sufficient pressure (and eliminate vapor lock).

Normal pumps hiss when working properly. A "buzzing" pump indicates impending failure.

You may be able to "feel" which pump is buzzing versus which one is operating normally. But an automotive stethoscope would give you absolute info.

Check the prices here on Fastlane, I seem to remember one being about $200. At your car's age you should change both, if one is failing the other one may be close behind. If you have the time and want to take the chance and save the money you could just change one. Then if there's still a problem swap them.

When mine failed it was temperature related. The fuel is cooled when the AC is on so I verified it by running the AC and listening.

Having problems when accelerating is a sign of a fuel pump heading south.