This is interesting. I've never seen a sight glass in an original HFC134a system. My '96 E doesn't have one.

If you have the digital climate control unit, you should be able to extract current values and trouble codes. Use the search for the procedure. On my E, I get the current values by holding the REST button for >5 seconds, then scrolling with the two AUTO buttons. The procedure to get the trouble codes is somewhat more involved, but still takes no tools.