You can jump the switch to test if it's working.  Probably not a good
idea to leave it jumped if's bad.  It's there for a reason, most
likely to protect the compressor.  On a '91 DX it's on the drivers
side someplace near the canister with the viewing glass.  

I tried jumping my sensor years ago to see if there was too little gas
for the compressor to work.  When jumped the compressor worked.  I
then filled the system (converted to r134a) and worked fine for a few
more years when I had to refill again.  It's not worth it for me to
fix such a small leak on such an old car.