I have also seen this problem. There was a heat strip installed, but the thermostat did not show any indication of it. When the builder was asked about it, he indicated that the heat strip came on when the unit went into defrost mode. I didn't really believe that explanation. Most heat pump thermostats will automatically turn on the heat strip when demand is high, such as when the thermostat is turned up significantly higher than the interior temperature (some of them when demand is only 2 degrees higher). So, if I find a thermostat like you are talking about, I turn it up by 10 degrees and then put an ampmeter on the wires to the heat strip or the wires to the air handler in the electric panel. 5KW strip will pull about 22 amps - so at least I know it works. The air handler will only pull a couple of amps without the heat strip on.