How to confirm if A/C system is discharged of pressure

Hello, I have just recently came back from mechanic and was told I need to replace my compressor because it has "hole" and is "warped". He did a evacuate and recharge to test it. He told me I need to replace drier (I assuming this is the accumulator too), and orifice tube. Along with flushing system. I am able to see the parts to replace and they don't seem hard to take off. I have an empty can of Freon with psi gauge. When I hook it up to low port of system it reads 0 while engine is on and a/c is full blast. Is this reliable indicator to take apart a/c components with out danger?

p.S. It seems the mechanic evacuated, recharged, and evacuated again. :-)