If you have modern type HP thermostat, there should be a light indicator that shows when your heat strip is activated. If your thermostat does not have that indicator, check your electric meter. When heat strip is activated, the meter wheel/disc should be spinning faster than you can see.

IMO

If you have an in-door/out-door thermometer, stick the out-door sensor down a supply vent. When the aux/backup is on, the temperature should be higher than when just the HP alone is running. However, if your heat strips are staged, this may or may not be true.

SR

Set the thermostat to Emergency mode. That will activate the aux heating and deactivate the HP. If it blows warm air, you're ok.

Good suggestion by weed...

Alternatively to weed's suggestion, you can crank up the heat by 5 or 10 degrees...this should also kick on the Emergency Heat.

Not always Cas20. I am sure there are some people that have thermostats with advanced settings and no idea about it. Some stats with outdoor temp sensing can have lockouts on the strips and pump at different temperatures. Right now its above the 35 degrees my strips are locked out at so they will not come on no matter what I set the temp.

Emergency mode is probably the easiest for sure test.