What is the pH of starch?
Starch is a weak acid, but a stronger acid than water.

If you make a slurry of starch and add sodium hydroxide the starch will react with the sodium hydroxide, the starch slurry will then change in viscosity. It normally becomes a lot more viscous in my experience. This is part of the method by which starch is alkylated. I have made moderate sized batches of benzyl starch from corn starch by first stirring the starch with sodium hydroxide and then treating the resulting paste with benzyl chloride and then heating it up.

If you were to get a very senstive pH meter you would see that a slurry of starch in water is slightly more acidic than water would be. An alternative would be to use a dilute sodium hydroxide solution to make the measurement, I will warn you that it can get messy as the starch can be transformed into a paste / slime.

MaxW is right in part, a pure starch will give no ash but zinc sulfate will dehydrate to anhydrous zinc sulfate unless you heat it very very hard.

Another method is to add iodine, many starches will form a deep blue / black colour with a small amount of iodine. Zinc salts will not.