Question on clutch M/C

How can I tell if a clutch master cylinder is bad? The guy I bought the car from told me that the M/C was bad, and he bought a used one from a junk yard but it was bad too. I have one of them that was just sitting in the trunk and I was just wondering how I would know if it is bad or not.

your not going to know untill you hook it up. i implore you to please just get a new one. its way to much work to gamble on a used one.

yeah, i think thats what im gonna do. Thanks.