It depends on where in your case you want to put it and which way you have it facing.
Fans push air in the direction of where the wires of the fan are mounted. Or another way is where the fan has three or four plastic bars, that run from the outside to the inside of the shroud. Also, sometimes they are marked with arrows and or actually have the words "air flow" written on them.
They push air from one side out to the other. Plug it in and feel it or look on the side and you may find 2 arrows at right angles to each other.
Hold the fan so that the blades are vertical. The up (vertical) arrow indicates which side should be up and the horizontal (left/right) arrow tells you the direction of airflow.
My old psu used to push air out. Looks like the new ones suck air into the psu if the psu fan is on the bottom and out the back.