A good rule of thumb - if it crawls out of the can/pouch/container under its own power, it is time to toss that particular ration.
Seriously, though, the Vietnam-era C-rations are now purely historical in value, unless you want to try a really nasty, prolonged way of killing yourself by eating them. I know a guy who still has some WW II C- and K-rations, just for curiosity’s sake.