The filter location varies from system to system, thus it is always handy to know where your filter is located. In most cases, there is a removable filter access door in the return air duct beside the furnace or indoor unit. This is usually located in the basement, attic, or crawl space.
For older systems, you can locate the filter inside the furnace beside the blower motor. There are also systems that have a central filter grille positioned in the ceiling. To reveal the air filter, simply swing the grille open.
Bear in mind that, most filters have a single direction; that is, the air flows through in one direction. There is usually an arrow that indicates the direction of airflow, pointing toward the air handler and furnace.
Occasionally filters will not have arrows. In that case, the best indicator is to look for the rougher side of the filter, as this is usually the part that accumulates dust, thus indicating that the other side faces the equipment.