How do I know which direction a furnace filter should be installed in? There is a nice little arrow marked airflow on the filter, however, I don't know which direction the air is flowing. Obviously, I failed to pay attention when I took the previous filter out.

Seriously though, why isn't there a great BIG arrow on the furnace itself?

Anyway, I apologize for a rather lame question. Thank you for your consideration.

Arrow points toward the air handler and away from the cold air return plenum.

Stecworld,
I commend you for asking. Probably 75% of the furnaces I work on where the homeowner changed the filter, it's installed backward. I routinely use a marker to write the filter size on the return plenum with an arrow showing airflow direction.