"an L-ruler or T square works, folding would probably work too. set one edge of your ruler alone the hipline or whatever horizontal reference line you are using and then draw your grainline against the vertical edge of the ruler."

"technically the hipline, kneeline and hemline should be parallel to each other, thus the grainline would be perpendicular to all of these but in the absence of other reference lines, drawing the grain perpendicular to the hem can work if it's a…"

"Usually the grainline is perpendicular to the hemline, assuming the pants have a straight hem. Or mark it up the center of the pant leg, this works too."