It's easy to tell when D76 is dead : it's when it's not some white powder any longer, but a kind of a light brown solid brick instead.
AFAIK (personal experience) D76 in modern supple pouches holds much better over time than D76 in the old type can (less air inside).
Last old type can of D76 I bought was in 1995, I wanted to use it in 2008 and it had become a brown brick.
As far as A and B components of Microphen and ID11 haven't oxidyzed they are good for use, and they are less likely to oxidyze than D76. It depends a lot on where they were stored (temperature, humidity etc).
Anyway given that a modern pouch of D76 (for making 1l of stock bath) will cost you €4.50 I wouldn't try to process some important pictures with 20 years old powder developers. But you can always test them on a test film roll.
If you want to hoard your developer, you must switch to Rodinal and films that like Rodinal, so bye-bye Tri-X and HP5+ then. 😉
