How to draw a depth buffer on a square in OpenGL ES?

Question

OpenGL

How can you "see" what is drawn in the depth buffer? The bottom line is to draw the depth on a square and use these "pixels" to calculate the distance to the point where there was a click on the smartphone screen (translating screen coordinates to world coordinates) on OpenGL ES 2 or 3, where it is not possible to read from depth using glReadPixels (or did I make a mistake?), if there is a more beautiful way to do this, then tell us about it, thanks for your attention!