94 catalytic converter how do you know when it needs replaced? need replaced

My muffler is shot so was wondering when I replace it if I should replace the cat too??
What is the life span or how do you test them to know what kind of shape it is in??
About 155,000 miles on truck I assume both are original.

There probably is some sort of flow test to check one, but likely not worth the effort.
I wouldn't replace it unless necessary, for one thing, they're $$.

If it is clogged, you'll notice it. One clue is if it starts struggling uphill where it didn't before.
If you have an O2 sensor before and after it, the system should see a problem a trigger a CEL.