Double Acting Baking powder has baking soda (sodium bicarbonate) AND two acids it in Single Acting only has one acid
You can use this property to test by mixing the unknown substance with some plain water (tap is fine). The mixture might start to bubble slowly, as one of the acids in baking powder will start to act when mixed with water and is now able to react with the base.
Double Acting baking powder also reacts with heat
If you gently heat this, there is another acid that will release when wet and heated. This will further react with the baking soda. This is so it will act slowly as the quickbread or whatever is baking.
Baking soda will only react with an acid
If your white powder in an unmarked bottle does not react under the above conditions, now test it with vinegar. This will confirm that it is a base (likely baking soda)