Question

For checking dryness of air which instrument is used

Answer 1

Air humidity meter or hygrometer

Answer 2

Dryness of air is actually measured by the hygrometer.

Dryness of air is actually the absence of moisture or humidity in the air and we measure the moisture and humidity with the help of the hygrometer.

So in another sense we can easily detect the absence of moisture or humidity in the air with the help of the hygrometer which means we can measure the dryness of the air with hygrometer.