How to systematically find my HVAC return air intake?
Houses from that era usually had 1 or 2 large floor grates as cold air returns or the return grills/registers were on the outside walls under windows. There has to be return air grills or you would not have any supply air coming out the registers. Are there any registers high up on the walls, these could be the returns.With the fan running, light a match or candle put it near the register to see which way the air is flowing, in or out. Follow the return air duct work, from the furnace, (the duct that holds the air filter), and see where it goes. If you find metal sheeting nailed to the floor joists in the basement, this would also be part of the return air system. Follow it to see where it goes. Where the metal stops, that is where the return grill/register is, above that location in the floor or in the wall. And if you use the basement for anything, storage or any use, you should have 1 or more supply registers and return grills to keep that area somewhat conditioned. One last thing is this house a 1 story ranch, split entry or a 2 or more stories?