How can I know if my HVAC is taking air from outside?
They're not very common in residential, but an outside air duct would connect to the return air duct. Trace out the return air duct and see if there are any smaller ducts attached to it. Or walk around your house and look for the opening the duct would connect to. It would most likely be near the furnace. As for your filter, you can get some foil tape from the hardware store to tape over the filter opening so no air is bypassing the filter.