When you make books, you need to know the direction of the paper grain so the folds crease well, the accordions lie flat, the pages don't spring open. Someone wrote to me last week, asking about paper grain of a specific paper: "Canson states that Mi Teintes has a variable grain." Did I know what that meant? I wrote back that my experience with Canson Mi Teintes is that it does have a grain. In this case "variable" might refer to the size of paper or paper all from the same batch. Mi Teintes papers of the size 19 x 25" all have the same grain, again, from my experience. I do not recall which way, and I don't have a piece at hand, but there are some tests you can do.
Bend the paper in the middle (but don't fold) and gently press on it, long way, then short way. Which way does the curl seem to get smaller? That's with the grain. Which way does it seem to resist you? That's against the grain.
Tear a piece: tear a strip lengthwise (down the longest side), then tear a strip widthwise (the shortest side): which tears more easily and straighter. That strip is torn with the grain.
Wet a piece: which way does it curl? It will curl parallel to the grain.
(I like that the acronym for these three also stands for "By The Way.")
Paper in a roll is grained long. Ever notice how your paper towels don't tear very well? I hate how I pull to tear it and it ends up scrolling across the floor. How unhelpful. The perforations are against the grain.
And how about the deckle edge on some papers? (The deckle is that wavy edge that looks like it has been torn.) Stonehenge, for example, has two cut edges and two deckle edges. This paper was not handmade: the pulp was fed onto a conveyor belt and little jets of water were sprayed at the edges to achieve the handmade look. It's grained long; in general, paper that has only two deckle edges is grained long. There are always exceptions, so it's good to check.
Handmade paper may have a non-discernable grain. It's very hard to tell. When it's made, the pulp is shaken in various directions, causing the fibers to lay this way and that. One way may be stronger. This is where you come in. If it isn't possible to tell, use it how you like. But test it, too.
Ultimately, you are the cook. From experience, you learn what the ingredients and spices taste like. You know how they should work. Then it is up to you to trust your own eyes and hands and not worry too much about the recipe, or the words the manufacturer uses. Experience comes from practice and testing. Those are our jobs.
Sometimes you don't want to reinvent the wheel. But sometimes you learn more that way. Then again, after you've tried, it never hurts to ask.