Like most products of living organisms, coffee beans are highly susceptible to aging. The moment a roasted bean is exposed to air, it immediately begins to grow stale and lose flavour. Even worse, some shop bought blends may even be stale before you buy them.
There are a few easy ways to tell whether your coffee beans are fresh based on appearance, smell and feel.
When coffee beans are roasted they are exposed to intense heat that extracts moisture from the heart of the bean while simultaneously drawing out oil-like substances that, then, coat the outside of the bean.
If you pick-up a handful of beans and they leave a residue on your hands, or you see a lining of residue on the inside of the bag, these beans are fresh for brewing.
Fresh beans are known for their intense aroma. If you can’t easily smell your beans, then they are likely to be well past their prime.
Freshly roasted beans give off a considerable quantity of CO2. If your bag of coffee does not include a valve, then your coffee beans are not actively releasing CO2 and are not fresh.
Place a handful of whole coffee beans in a ziploc bag. Press out the remaining air before sealing, let it sit overnight, and check in the morning. If the bag appears to be inflated due to the release of CO2, then your beans are fresh. If the bag remains flat, then your beans are past their prime.