The borders of the plates are drawn after the scientists noticed and studied some of the natural phenomenons, like volcanic activity, mid ocean ridges, riffs, movement of the continents, earthquakes... Once these were studied, and was figured out that they are all consequence of the crust layer being broken to multiple pieces and the pressure from the mantle layer from bellow, the scientists used this activities like indicators where the plate's borders are to create a map of all of them (studied so far, at least) and draw the lines between them.
