It can't be done. The human eye cannot see X-rays. So if any effect was caused by X-rays, a human being would not be able to see the skeleton.
The one range of frequencies where this could potentially work (so it could be seen by people) is in the visible light range, but there's a problem there--even in those areas where light could potentially pass through the human body, it diffuses and scatters. So what you end up with is light scattering and shining through the flesh, but too scattered to show where the bones are.
I know, it makes a neat visual effect--see the bad guy get zapped, and their skeleton shows through. It's been done in everything from cartoons to the movie The Sum of All Fears. But it doesn't actually work in real life.