When to replace an exhaust

I'm running a standard TV175 S3 through a standard style exhaust of unknown origin. When I last replaced an exhaust - in the late 60's - I seem to remember that the test was to weigh it and compare it with the weight of the readily available (through any dealer) standard replacement. There was a "suggested" weight at which the old one was considered past it.
As various people now sell the standard design (which I'd like to stick to) they are bound to be different weights so this is presumably no longer the way to test it. So how do I know when the time has come and what suggestions do you have for a replacement if/when needed?
As various people now sell the standard design (which I'd like to stick to) they are bound to be different weights so this is presumably no longer the way to test it. So how do I know when the time has come and what suggestions do you have for a replacement if/when needed?