mam, how can we differentiate between position time graph and velocity time graph?

There are many ways to do so.
First, if p-t graph is a straight line and passing through origin means that the body is moving with uniform velocity and on v-t curve it will be parallel to time axis.

If p-t graph is a straight line parallel to time axis means velocity is zero after that instant, on v-t curve it will be a line coincident to time axis as velocity is zero.

And the Third and least common chance if the p-t graph is the type of polynomial ie A quadratic curve or a cubic curve, just differentiate the equation and draw a curve from the resulting one and that will be velocity-time graph.