Is it possible to use the appearance of hydrates to determine if there is moisture in a room?

Question

Yes, it is possible in some cases. For example, CoCl2 is blue in the unhydrated form and pink in the hydrated form. As a kid, I used a water color brush to "paint" a solution of CoCl2 onto a fish from a coloring book. I hung the painted finsh outside. The fish was blue when the humidity was low; it was pink when the humidity was high. I hope this helps.