How to tell which way your fan is blowing

Spend enough time around PC builders and PC building forums, and you’ll learn pretty quickly that the direction of a case or CPU cooler fan matters. When a fan’s positioned for intake, the intent is to pull in cool air. But when a fan is set as an exhaust fan, the idea is to expel warm air away from the case or cooler. Both options are useful depending on where the fan resides in the build. So how do you tell at a glance which way the fan is blowing, especially when even seasoned PC builders...