Question

how to say that the body in in rest or motion?

Answer 1

We do say a body is at rest when it does not change its position from a fixed point (which is at rest) with respect to time, even on the application of force. A body is said to be in motion when its position (from a fixed point) changes at regular or even at irregular intervals of time.

Answer 2

we can say that the body is rest if there is no change in the body with respect to time
we can say that the body is in motion if there is a change in the body with respect to time