It's recommended to replace both at the same time if one or the other is replaced. Check in the footwell against the firewall. If you see brake fluid leaking down, it's the clutch master cylinder. If not, unbolt the two 8mm bolts on the bottom of the clave cylinder and remove it with brake line still attached. If the little push rod slides completely out (like mine did), this could indicate broken seal and brake fluid leak. Definitely replace both either way.