I'll bet the arrows indicate to show that pieces on the edge of the block are on the straight of grain. That would be all that I would worry about at any rate.
With scraps you can determine straight of grain, and even WOF and LOF.
To determine straight of grain, eyeballing it is really good enough to insure the straight part of the edge of the block will be on straight of grain.
To determine which way is WOF and LOF ... give the piece of fabric a little tug on the straight of grain in both directions (ie horizontal and vertical). The one that stretches a tiny bit is the WOF, the one that doesn't have any give to it is the LOF.