It can be difficult to gauge the freshness of dried herbs simply from looking at the bottle on the rack in your spice cabinet. Ideally, you should just periodically replace the herbs in your spice cabinet as just about everything will lose flavor over time. The easiest way to see how fresh things are is to smell them. If, for example, you dried basil or rosemary has little to no scent, it is likely that it is past its prime. It could even impart a slightly musty taste to your food, as you will have to increase the amount you're using if you want to get any flavor into your dish.
The best bet if you are in doubt is to replace the product in question, either with something fresh or a fresh bottle of dried herbs. If you find that you are often replacing full jars of spices, consider switching to fresh herbs for those ingredients. There is no sense in buying a new bottle of dried rosemary/ thyme/tarragon, etc. every year when you only use it for one dish. Buy the fresh herb on that day, get a better flavor in the finished product, and don't worry about it again for another 365 days.
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