560sl A/C Compressor Test

Hello!!
Yesterday my air conditioner was blowing ice cold air. I turned off the car, filled up with gas, and when I started it back up... the cold air was GONE!!

How do I test the compressor to see if it's locked up? With the engine off... I can easily turn the part of the compressor that's in front of the belt pulley. Is this good?

I can't get the system to "suck" anymore freon (R134a) inside, which leads me to believe it's the compressor.

What's an easy test to see what happened? Is there a fuse other than the one inside the passenger footwell?

Please help... it's starting to heat up here in Florida.

Thanks!