How to distinguish nutmeg freshness? Does nutmeg come with a good-before date? How long can nutmeg stay good? Is there a way to 'see' (observe) how old the nutmeg is?

I'm asking because I bought a bag of nutmeg seeds and one was moldy. Maybe just bad luck, but it made me wondering about the freshness of the rest, and how that must affect the flavor in a negative way.

According to the Wikipedia, freshly ground nutmeg has a psychoactive substance, but I guess that must be from fresh nutmeg. So, I'm looking for a way to distinguish between potential poisonous nutmeg and stale nutmeg.
Simple: grate it.

If you grate the nutmeg using a microplane, it should have a strong, distinctive odor you can smell from a couple feet away.   If it doesn't, it's lost its flavor, throw it away and buy new.

It's fairly likely you're going to end up replacing it.  I've never seen nutmegs get moldy before, and I live somewhere with 80% humidity.  It would have to be stored in some very poor conditions to mold.