You can tell what side to shade because you test the point (0,0) you put 0 in for x and 0 in for y. If the answer comes out right than it the side you want to shade in with (0,0)
Ex. Y >_ 2/5x-2
0>_ 2/5 (0)-2
0>_ -2
This is correct because 0 is greater than or equal to -2.