Finding paper grain without guessing
Paper fibres line up in one direction as the sheet is made. Folding along that direction is easy and clean; folding across it cracks and fights you. In a finished book the grain of every sheet, board and cloth should run parallel to the spine, or the covers will warp as the seasons change.
1. The bend test
Gently curve the sheet over without creasing it, first one way and then the other. It bends more easily, with less springback, when you are bending along the grain.
2. The tear test
Tear a small strip from a corner in each direction. Along the grain the tear runs fairly straight; across it the tear wanders and leaves a ragged edge.
3. The damp test
Lightly dampen a small offcut on one side. It will curl into a tube, and the axis of that tube runs parallel to the grain. This is the most reliable test for heavy papers and thin boards.
Mark it immediately
Once I know the grain of a sheet, I pencil a small arrow in the corner before anything else happens to it. Offcuts in particular lose their history quickly, and a drawer full of unmarked scraps is a drawer full of guesses.