How do I know which way I put in the Air Filter in the air vent?  

Luckily most air filters have an arrow on them. You will always want to set the filter so the arrow point inward towards the air duct and HVAC system. Your system sucks the air in, and that’s what the arrow specifies.

If the filter does not have an arrow, then most pleated filters have a metal grid on the back and you’d want to set the filter so the metal grid is facing inward, so you will only see the filter pleats.