…often, the trees will tell you. Check out the wind-sculpted spruce trees above: The way the branches have grown tells you that from this perspective, strong winds usually blow from the left of your screen to the right.
Why does that matter? Well, winds do shift — but if you know which way the wind usually blows, it gives you a good idea of the best orientation for your tent (the small, low or narrow end points into the wind). If you’re dayhiking, seeing trees like this 1) warns you that the area you’re in is subject to frequent, strong winds and 2) gives you a clue about whether you might end up with the wind in your face on the return part of your hike.