Does x-ray lead paint detection find lead underneath latex topcoats?
The short answer is Yes, X-ray lead paint detectors can 'see' lead through top layers of non-lead paint.
But it depends to some degree on the thickness of the layers of paint as well as the type of X-ray detector 'gun' used.  

There are 2 types used for lead detection: K X-rays and L X-rays.
The K X-rays have higher energy than the L, which means that they are less attenuated by layers of paint.  

So a K X-ray lead detector will be better able to detect lead in lower layers of paint than an L X-ray detector.  

If you're concerned that you might get a positive reading from an X-ray lead detector, then you'd probably be better off having a chemical lead test done instead, since this should only be able to find lead in the surface paint layer.