a quick way is to find the rpm of the motor, then multiply that by the circumference of the drive wheel, it should tell you how fast the belt is moving and therefore the object on it.

so if the motor spins at 40rpm and the drive wheel has a circumference of 12cm, then the velocity is 480cm per minute or 8cm per second.

An accurate way of finding how fast the motor is turning is to use an encoder wheel on it