If you need somthing more precise than our regular moisture detection paper consider these test strips which register 20-40-60-80% relative humidity.
Wait at least 15 minutes before recording the humidity level. The vial is lined with dessicant and the strips need time to equilibrate. The 20% will change first, followed by the others. Note that these pads are reversible, changing color to white - pink and back to blue repeatedly, according to ambient humidity.