You can know if your house has negative air pressure by observing physical signs around doors, windows, and appliances and doing simple DIY tests. Common signs include exterior doors that are hard to open or slam shut on their own, whistling or howling sounds around windows and doors as outside air is pulled through cracks, cold drafts near electrical outlets and baseboards even when windows are closed, fireplace or wood stove smoke that comes back into the room instead of going up the chimney, and bathroom exhaust fans that are noisy or have flapping dampers. You may notice lingering cooking smells that don't clear, increased dust, and condensation on windows as humid outside air is pulled into walls. A dangerous sign is backdrafting of gas furnace or water heater with soot around vents or carbon monoxide detector alarms, which requires immediate professional attention. Simple tests include the tissue test, close all windows and doors, turn on all exhaust fans, dryer, and range hood, then hold a tissue near a slightly open exterior door, if it is pulled inward into the house, you have negative pressure. You can also use a smoke pencil or incense stick near door cracks and watch smoke being sucked inward. For accurate measurement, hire an energy auditor or HVAC technician to use a manometer that measures pressure difference in pascals, they can also perform a blower door test. If pressure is more negative than -5 pascals with exhaust on, you likely need makeup air solutions.
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