77 911 with orig York compressor - AC question

[Kimos]

I skimmed through search results and assume its freon low but wanted to ask.
Have a 1977 Turbo Carrera, she rarely gets out of the barn as I have others to "drive". The AC system was gone through about 2+ years ago - all OEM York comp etc . . .We replaced hoses as need and the system was vac down and recharged with R-12. I used the AC on a rare drive in the spring (this one is a keeper I have had for years and thus a garage queen). The AC worked fine in the spring as it always has.

Now the problem. Having not driven her in a month or so thus I decided it was time and took her out. This time however the AC wont kick on. The compressor not engaging.

I assume that freon has reduced to a low level and limit switch keeping it from coming on? Do the yorks have such a switch?

A bit rusty re AC (cant remember everything I guess) I have recharged the systems on my others i.e. SC in the past and still have manifold set but it has been a while. Best method to check pressure to see if too low? and/or do I put a can of R12 in (have one and access to more) to see if compressor kicks on?

Any assistance greatly appreciated

Cheers!