Is there a simple way for me to check that my H5i heater plate is working properly?

Yes. To confirm H5i function, complete the following steps;
1. Disconnect ClimateLine tubing and remove the water tub.
2. While the S9 is NOT running, place your hand on the heater plate to assess the temperature of the H5i heater plate. NOTE: Do NOT place your hand on the heater plate if the H5i has been running in the 10 minutes prior to this test as it may still be hot.
3. Set the humidifier output to 6.
4. Place the unit in warm-up mode and allow it to run for 1 minute.
5. Place your hand above the heater plate (without touching it) to confirm an increase in heater plate temperature. Don’t place your hand directly on the heater plate as it will be hot.
6. Press the dial for 3 seconds to enable cooling down mode.