There are a few signs that can indicate whether your pool heater is working properly:
Check the pilot light: If you have a gas heater, make sure the pilot light is on and burning steadily.
Check the temperature: Set the temperature to the desired level and monitor whether the water is heating up to that temperature.
Check for leaks: Inspect the heater and its connections for any signs of leaks, such as puddles of water or hissing sounds.
Check the pressure: If you have a pool pump, check the gauge to ensure the pressure is within the normal range.
If you notice any issues or suspect your pool heater is not working correctly, contact a professional for an inspection and repair.