[kinda obvious]In games without a debug viewer, you can't easily find the graphical X Axis. So the way to find this is rip an image on one side and jump over him capturing the same frame. You can now find the exact axis by finding where the sprite flipped.[/but I've never seen it discussed before]
I seem to have forgotten to mention that you don't want to have the screen scroll at all or you ruin aquiring the axis through this method. So doing this in the corner and making sure that there is no chance that the trajectory of the jumping character won't push the character you want to find the axis for is strongly recommended.