The grain direction of a sheet is usually the second dimension of the paper, meaning a 22x30 sheet would be grain long whereas a 30x22 sheet would be grain short.
This is obviously not always the case but usually a quick identifier. The best way to tell would be to fold a piece of paper. When folding against the grain you will feel more resistance then folding with it. Most often you will also see cracking at the point of fold where the fibers actually separate in the sheet. When dealing with Mould Made papers the grain will not be as pronounced so it will be harder to tell. This paper making process allows for the fibers in the sheet to lay in a more random pattern versus a Fourdrinier paper where the fibers are pushed in a direction as it is produced. This means the grain direction won't be as relevant with folding a Mould Made sheet such as Arches Cover
This is obviously not always the case but usually a quick identifier. The best way to tell would be to fold a piece of paper. When folding against the grain you will feel more resistance then folding with it. Most often you will also see cracking at the point of fold where the fibers actually separate in the sheet. When dealing with Mould Made papers the grain will not be as pronounced so it will be harder to tell. This paper making process allows for the fibers in the sheet to lay in a more random pattern versus a Fourdrinier paper where the fibers are pushed in a direction as it is produced. This means the grain direction won't be as relevant with folding a Mould Made sheet such as Arches Cover