Suppose there are two observers, they each own a light clock which were synchronised in the beginning and are moving relative to each other. So each of them accuses that the other person's clock is running slow. Now they see a concrete event, like someone clapping or a bulb shining and note down the reading on their clock and send each other the information, now if the two pieces of information were different, which means one of the clock was actually running slow, so they will know who was moving actually.
If the readings are same, then how will you justify Lorentz-Transformations for time ?