Force diagrams like the one shown depict the direction and strength of forces by vector arrows. Longer arrows mean stronger forces. One can determine if the forces are balanced by looking at the arrow size. Equal length arrows in opposite directions mean the forces are balanced. Such situations are consistent with constant speed. Unequal-length arrows in opposite directions mean the forces are unbalanced and the object will accelerate.
 

When considering how an object will move - whether it will accelerate or move with a constant speed and direction, it is important to compare the strength of the forces that act on the object. Forces, when unbalanced, cause objects to accelerate. And if an object accelerates, one can be assured that the forces acting on it are unbalanced. On the other hand, a balance of forces is consistent with an object maintaining a constant speed.

If an upward force is bigger than a downward force, then one can say that the unbalanced force is directed upward. This would cause an upward-moving object to speed up and a downward-moving object to slow down. 

And if a downward force is bigger than an upward force, then one can say that the unbalanced force is directed downward. This would cause a downward-moving object to speed up and an upward-moving object to slow down. 

Finally, if the forces acting upon an object are balanced, then the object neither speeds up nor slows down. It maintains a constant speed.