Suppose I am sitting in a car moving with constant velocity, and all windows

blocked(i cannot see what is going on outside), with no speedometer working.

i dont know whether the car is at rest or in motion. How will I find it out?

As Mr. kaushalsapre  has very rightly remarked that car moving with constant velocity is an Inertial frame of reference and hence, according to the law of Relativity, " in an inertial frame of reference laws of physics are exactly the same", i.e. all experiments conducted in inertial frames will give exactly identical results and thus it is not possible to find whether the car is moving or not. However, an accelerating body can be taken as non inertial frame of reference in which the results of experiments conducted will be different. for example in a car accerlerating with some acceleration 'a' the string to which bob is attached will not point in the vertical direction but it will subtend some angle with the normal drawn from the point of suspension. Earth is also an inertial frame of reference though it is rotating but rotational velocity is so small that it can be ignored.

thus, frames at rest or moving with constant linear velocity are inertial frames of references. Whereas rotating and accelerating frames are non inertial frames of references.