1. Feel and look
Run your hand around window frames, door edges, outlets on outside walls, baseboards and the attic hatch. Look for daylight around doors and for dirty streaks on carpet edges or insulation, which show where air has been moving.
2. The smoke test
Close all windows and exterior doors, turn off the furnace, and run the bathroom and kitchen exhaust fans to pull air in from outside. Hold a lit incense stick near likely leaks; where the smoke wavers or blows sideways, air is coming in. Keep it away from curtains, and don’t do this near a running gas appliance.
3. The paper test
Close a door or window on a sheet of paper. If it slides out easily, the weatherstripping isn’t sealing.
4. A thermal camera
A phone-attached thermal camera shows cold air leaks as dark streaks around frames, outlets and ceilings on a cold day, and missing insulation as cold patches on walls. They’re affordable now, and you can sell one on afterward.
When to call a pro
A home energy audit with a blower-door test depressurizes the house and measures exactly how leaky it is, and shows where. Many utilities offer them free or at a discount. It’s worth it before spending on insulation, because it tells you where the money matters.