Answer: There are multiple ways to know some object velocity:
One is knowing the distance that it travels in some amount of time, you can do the quotient (because velocity is a measure of distance over time) and obtaining the mean velocity of the object in that period of time.
Other way is knowing the acceleration of the object for a certain time, and the initial velocity of the object, we know that if we integrate the acceleration over time we get the velocity, and when we integrate, we must add a constant of integration, which is the initial velocity of the object.
