Thermostat for my heat pump system in my home has died. I think that some heat pumps energize the reversing valve in heat mode and some energize the reversing valve in cooling mode. Question is : "When replacing a thermostat for a heat pump how can you tell which type of system you have and does it matter when buying a stat for a heat pump. Will they work for either design?

It depends upon the manufacturer of equipment as to the reverse mode they use. How do we know what brand of equipment you have. I gave up mind reading when I retired. LOL