The 1 Rule of Binding
In this method completely fold the paper both ways, the direction with the cleanest fold is the direction of the grain. It is noticable when its been folded the wrong way, there will be small crinkles and folding is harder.
Tear the paper in both directions quickly. The rip will be fairly clean and stay straight if it has been torn in the direction of the grain. If you tear paper against the grain the rip often curves to one side and is messy
If you wet the paper the paper will curl in the direction of the grain. Wetting just the corner is sometimes sufficient.