How to tell which edge was touched in Scratch?

Make 4 sprites, all thin lines of any color you want. Two vertical, and two horizontal. Place them at the edges of your project. Add a start block followed by a set effect ghost to 100 block in each of these four lines. This hides the line when the project is played. If you name the lines top, bottom, left, right, it's easy to test which edge a different sprite touches, because you can have blocks run after one sprite touches another (the edges).

The whole project is 360 tall and 480 wide, with 0,0 being in the exact center. Always.