What strategy can you use to find if a graph is proportional?

If the line is straight and goes through the orgin

A proportional relationship between two quantities is one in which the two quantities vary directly with one and other. If one item is doubled, the other, related item is also doubled. Because of this, it is also called a direct variation. The equations of such relationships are always in the form y = mx , and when graphed produce a line that passes through the origin. In this equation, m is the slope of the line, and it is also called the unit rate, the rate of change, or the constant of proportionality of the function

That they are numbers and related to each other.