If you are talking about a boundary line on a coordinate plane, the best way to determine which side to shade is to start by selecting a coordinate point (x, y) that is not on the line.
Then, substitute those values into your inequality. Simplify the inequality. If the resulting statement is true, then shade the side with your point. If the resulting statement is false, then shade the other side.
Testing a point like this shows the side of the inequality that is the solution and needs to be shaded.
