Cumin seeds should be khaki in colour. Ground cumin powder feels oily (cumin seeds have a high oil content). Black cumin seeds are dark brown and slightly smaller than regular cumin. Be careful buying ground cumin - it should be khaki rather than brown. Dull brown powder will probably have been adulterated with cheaper coriander seed.
Whole and ground cumin seeds should be stored in a well-sealed airtight container away from light, heat and humidity. Whole seeds will last about 3 years, but powder should be used within 12 months of purchase.
Tips & Tricks:
Due to the relatively high oil content, ground cumin can be dry roasted in a pan quite easily without burning. They are not to be confused with nigella seed (kolonji) which is often sold as black cumin.
Blend your own spices to make Garam masala (hot mixture) by grinding together roasted cumin and coriander seeds, black pepper, Indian bay leaves and smaller quantities of cinnamon, cloves, cardamom seeds and nutmeg. Use in spicy soups and curries.