I go by taste. For example when I make pumpkin pies I always taste to see if the flavor is right. If i see that I have to put in more of any of the spices than usual I assume they are weakening. I go ahead and use but add more. That way I'm not totally wasting it. Then I know to add it to the grocery list. Now I don't buy the large size spices as a rule. I buy smaller and more often.