How can you tell when an object changes direction on velocity vs time graph

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!

On velocity vs time graphs, if an object's velocity goes from positive to negative, or negative to positive, then it has changed directions.

A positive velocity means the object is traveling in the positive direction and a negative velocity would mean it has turned around and begun traveling in the opposite, or negative direction.