The horizontal grain is slightly stretchier than the vertical grain. Go to fabric store and practice on new, bolted fabric. Pull on the known vertical and horizontal grains until you can differentiate. Vertical grain is equivalent to the straight-of-grain marking on patterns. All fabrics react the same way, even knits. Knits do, however, also have an up or down to them. For up-down orientation, look at construction/ orientation of the knit stitches. Observe loops in a bulky sweater to know which way is UP. Some fabrics, like corduroy, velvet and flannel have "nap". Run your hand lightly over the vertical grain and you will notice the surface 'fuzz' moving up/down. Usually a napped fabric's 'nap' should run, lay, downward. Make sure that the 'nap' orientation is consistent or the garment will look weird.
