Use a digital hygrometer
Use a smart thermostat with humidity display
Check a weather station with indoor sensor
Use a hygrometer app paired with a sensor
Read the humidity on an HVAC control panel
Compare with a psychrometer
Check a dehumidifier or humidifier display
Place the sensor away from windows, vents, and direct sunlight
Wait a few minutes for the reading to stabilize
Aim for indoor humidity around 30% to 50%