How can we determine whether an object is at rest or not?

(N/A) If an object changes its position with respect to another object (the observer or a reference frame),it is said to be in motion with respect to that object; otherwise,it is said to be at rest.
For example,if you are traveling in a bus moving with constant velocity and a book is placed beside you,the book is stationary with respect to you.
However,the same book is in motion with respect to a stationary observer standing on the road.
Thus,the motion of any object is a combined property of the object and the observer. There is no absolute motion without a defined observation position or reference frame.