Which direction is the airflow through my furnace?
That's an upflow unit, so the air flow would be in the direction of the red arrow. The air flows in through the lower side, up through the heat exchanger/evaporator coil, and out the top of the unit.

The first clue, is that the filter will always be where the air enters the unit. Secondly, the blower is typically on the inlet side of the furnace/air handler.