Hot water! Put a teaspoon of it into a cup, and pour very hot water-about a dessertspoon full on it. If it foams, it's good to go!
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Second Lemonie's comment. I've been using the same can for a decade.
As an inorganic material, baking powder doesn't lose "freshness" like perishable foodstuffs. The worst that will happen to it is that it will get damp - which should be noticeable from it's appearance. If it's not a dry and free-flowing powder it's "gone-off" a bit. If it's a solid-lump it's "gone off" a lot.