I used to use a steel ball on the frame to level my old printer when it was set up at a new location- it was much more sensitive to tilt than a bubble level. If the X axis uses a piece of t-slot you can put a steel ball on it and it will be a very sensitive tilt sensor. Then all you have to do is detect the ball, either optically, magnetically, or using a piezo element.

In a dual motor set-up the X axis won't usually tilt while the machine is printing, so the Prusa technique of running the X axis to the top of the Z axis and a little beyond to force the Z motors/screws to sync works pretty well.