Well, first things first, check out the age of your furnace. Most of these units usually live to be about 15 to 30 years old. If your furnace is pushing those numbers, it might be a good idea to start planning for a replacement. Also, keep an eye on those energy bills. If they've suddenly spiked, your old buddy the furnace might be losing some efficiency. And remember, if you're seeing excessive dust, dirt, soot, or rust around your furnace, it could mean trouble. You don't want it coughing up those particles into your home.