Wind is one of the most crucial variables in any kind of big game hunting. It helps level the playing field between a hunter with a scoped rifle and the game animals being hunted. This is not novel information. Any hunter who has consistent success in the field knows this. I have tried a couple different techniques for keeping track of the wind. Here are a couple.
The most simple and obvious is to just stay cognizant of it. It is amazing how slight of a breeze you can sense if you just pay attention to it. However, there are methods that make wind direction slightly more obvious to the hunter.
The next most simple method is to just pick up some dusty soil and toss it into the air. The problem is that the perfect kind of soil for this isn't present everywhere. I have been in some places where there was no bare soil or the soil was to moist and it just wouldn't work. The nice thing about this method is that you don't have to carry any extra gear.
There are commercial products out there that will determine wind direction. Most of them are simple enough; they're normally a small squeeze bottle filled with a white powder that floats on the wind when sprayed into the air. You can easily make one of these by going to your local grocery store and getting yourself baby powder and a small plastic squeeze bottle.
Tooth floss can also be used as a good way to check wind direction. It can be tied to just about anything (rifle barrel, tree stand, backpack etc.) with a long trailing end hanging off. Another tool that most hunters already carry that can be used to check wind direction is a lighter. You can light a lighter and and watch the flame dance with the breeze.
With the sense of smell that most game animals possess, hunters need to be able to identify and keep tabs on wind direction. Use these methods to keep the wind in your favor.