How can I tell if this is baking soda or powder?
Baking powder contains starch, which is insoluble.

Baking soda is completely soluble

Take a small bowl, and put 1/8 tsp of the substance in the bottom.

Add water.

If the substance is bicarbonate of soda, the solution will be completely clear.

If it is baking powder, a cloudy/powdery residue will remain.

(You can also use excess vinegar which gives the same result, and fizzes in a pleasingly sciencey fashion...)