My cat started leaking around the pipe-to-cat weld at about 45k miles. Noticed as the exhaust note became rorty and if you held the back of the hand to it, you could feel the escaping gas.
My wizard local garage welded it up very successfully.
I guess other indicators of a failing cat would be on the emissions test during the annual MOT test. I have also known of the innards of a cat falling loose, blocking the exhaust with a corresponding extreme power loss.
