I usually write/note the date that I open sealant tubes (Sharpie scribble on the squeeze key). If it's more than a year old, I proceed with caution. Usually, I remove the cured plug in the tube opening and assess what's underneath. Squeeze out a portion on a card and see if it behaves as expected (skins over, cures, etc.).
9/10 of the time the product is still good.