what are the first symptoms of worn A/C clutch

[Konstantin Bogach]

Today I've heard periodic screechy sound  when A/C is on.  First thought
was belt.  Maybe it is the belt.
But I want to be prepared for the worst.  Bentley does not have anything
on clutch condition checking.
How to figure out whether a clutch  is worn out?  Both clutches from my
2 cars look the same and one is still on my workbench.

Thanks.
Konstantin.