In my opinion, that is one humungously fantastic question, that is not asked nearly often enough.
The answer is:. You can't ! None of the so-called "velocity/time" graphs shows any information about the direction of the motion, and there's no way to tell if it changes, from the graph.
There's no difference between the "velocity" graphs of an object in straight-line motion with constant speed, and another object in uniform circular motion. Both graphs are horizontal lines.
A "velocity" graph can't show direction OR displacement, and the way I see it, such a graph is not entitled to use the 5-dollar word 'velocity' at all. It is simply a graph of SPEED vs time, and no more.
So there !
