heat pump question - condenser to resistance heat?

Easiest way I have found will be the smell. The first few times you hit the heater strips they burn any dust off of them. Don't know how else to describe it other than "it smells like heat".

The more precise answer would be to ask what you have for a heat pump and thermostat. That will help determine what activates the heater. Some units base it on the delta between room temp and setpoint, or you might have to activate the heater yourself (the thermostat would have an " Emergency Heat" setting)

It depends. I have a Trane and the thermostat is what calls for backup heat. When the setpoint gest to be about 2 degrees above the actual temperature(meaning the heat pump can't match the load) the resistance heat kicks in. The compressor continues to run. It will turn on if I raise the setpoint by more than two degrees. It also runs anytime the heat pump goes into its defrost cycle. The information should be in the thermostat manual.

Either play with your thermostat, or try to google up the manual for your thermostat.

In southern PA, unless the HP is ancient (say 1900s), you should never 'lock out' the compressor, and just let the 'aux' or strips help out as needed as a 'second stage'.

That said, many HP installers set up the compressor to lock out at some high temp, like 30°F, to avoid callbacks, and the difference can be several hundred dollars a year extra in utility bills.

It is worth your time to research this issue and get it right.

AS for your original question....the stat will prob say 'aux' or 'second stage' when it is running...sometimes the air from the registers gets noticeably hotter, or the speed of the blower steps up (and gets louder). Or you can just switch off the breakers to the aux strips, and see how cold it has to get outside before your compressor runs 100% and you start to get cold inside!

My thermostat has a big red light that comes on when it calls for aux. I call it the money light.

I got a pdf of the manual here: http://www.manualslib.com/manual/141466/Robertshaw-9420.html

Looks pretty primitive for 2 stage control....if the temp gets 1° below setpoint, it turns on first stage, if it gets 2° below, it waits 20 mins and activates second stage. The temp diffs and delay are adjustable.

It looks like it displays one 'flame' icon in first stage, and two 'flame' icons when the aux is being called.