How can one determine if their cat is clogged?

Does it need to be removed and visually inspected to see if any light passes through or is there an on vehicle test.

My 87 has 163k on it and I'm wondering if a new cat might wake it up some. I put on a 2 1/4" cat back with magnaflow on when the stock exhaust rotted and fell off and this has been a HUGE difference but the 4runner is heavy enough and since I'm at altitude I need all the power I can get.

'87 4Runner SR5
'89 4x4 shortbed pickup Soon to be parted out