pool heater

What is everyone’s method on testing pool heaters? Because it takes a long period of time for them to heat up a pool.

You could feel/test the discharge pipe of the heater. If the heater is working, the pipe should be warmer than the inlet pipe

On a heat pump it will discharge cold air. On a gas heater you should know if it is working. With Solar we just look for leaks and check the controller/valves.