How to tell which way a fan blows
Standard fans push air out through the side with the sticker and the support struts. Noctua puts it simply: with the sticker facing you, the air blows toward you.
Reverse blade fans flip that, so the clean blade side faces you while the fan pulls air in. Phanteks describes its regular D30 as moving air front to rear and the reverse version rear to front. That's why the sticker rule doesn't work on them.
Frame arrows settle it for any fan. Most fans have two small arrows on the frame: one shows which way the blades spin, the other shows which way the air flows. As Corsair explains, the airflow arrow is the one that matters.
Which way each spot should blow
In most cases, front, bottom and side fans pull air in, while top and rear fans push it out. Corsair recommends exactly this for its 6500D Airflow case.
There are exceptions, like a radiator or a case built for a different airflow path. Check our case cheat sheets for the layout of your case, or test both fan directions in your own case with our sister site's airflow simulator.
Still not sure? Try the tissue test
With the PC on, hold a thin strip of tissue near the fan, outside the case. If it's pulled toward the case, the fan is an intake. If it's pushed away, it's an exhaust.