When the forces acting upon an object are balanced, the object will maintain its state of motion; that is, its velocity remains constant. If the forces are unbalanced, the object will either speed up or slow down. The reverse logic is also true: if an object has a constant velocity, then the forces are balanced. And if an object is speeding up or slowing down, the forces are unbalanced.
 

The slope of a position-time graph is the velocity of the object. By observing the slope over the course of time, one can tell if the object is speeding up, slowing down, or moving with a constant speed. If the graph shows a straight line - that is, has a constant slope - then the object has a constant speed. And as such, the forces must be balanced.

Alternatively, if the line is shown to curve, then the slope is changing and the object is either speeding up or slowing down. A line that becomes steeper and steeper over the course of time is consistent with a speeding up motion. And a line whose slope is getting less and less steep over the course of time is consistent with a slowing down motion. In both cases, the forces will be unbalanced.