I use the same criteria I do for medicines.

1) changing of color, changing odor (sniff test),

2) mold growth.

I find that spices with green herbs in them, the green will go off color when they get old. (cilantro, parsley) Keeping them in the dark may help.

I find that curry and ginger keep a LONG TIME.

Things mixed with garlic pieces...like MrsDash attract moisture and go bad quickly if unused. The red pepper one seems to keep better (based on my vacation home left overs where there is dampness).

I have some freeze dried onion and garlic upNorth that keeps for years! (I have to seal it up with a zip lock after opening.)

Exotic mixtures--like steak rubs, Thai spice etc...do not keep well for me. I throw those out often.

I also seal up the black pepper in our little grinder. It keeps several seasons that way.

I have no trouble with nutmeg or cinnamon. They keep well without special attention besides screwing the tops on tightly.

So things that are hygroscopic (attract moisture) are the most problematic.
We notice that vanilla extract with its high alcohol content remains fresh for a LONG TIME... years! The alcohol deters bacteria. If it gets cloudy, I'd toss it.

If you save the little canisters that come in some RX drugs, you can put those in the spices. You can reactivate them with a blow drier. But this takes patience.