Just heat your oven to 400 F (or 200 C), sprinkle some baking soda on a shallow pan, and bake it for about half hour, until it changes composition. You should also stir it up occasionally, just so that it bakes more evenly.
So how do you know when it changes into washing soda?
That part takes a little more work; just a closer, watchful eye. Once you know the differences between the 2 sodas, you’ll be able to tell in no time.
Baking soda is powdery, crystallized, and clumps together.
Washing soda is more grainy, dull, and it doesn’t clump as easily.
You can see the difference below: baking soda on the left, and washing soda on the right. See how the baking soda likes to clump together, and the washing soda is more flat and spread out?
And a word of caution:
Washing soda should be handled with care, as it is very caustic. Always wear gloves if handling it directly, and never inhale the dust particles directly.