One thing you can look for, on your thermostat on the wall, it will have a light (usually red or orange) that comes on when the "emergency heat" comes on. The emergency heat is your electric strip heaters. It is normal on a really cold night (below 28F or so) for this light to come on for 5 - 15 minutes as the unit is in defrost. If you notice that light staying on for half an hour or longer and doing it every time your heat cycles on, it is probably a good idea to have your unit checked out.