Just so you know, x-rays show the diaphragm so-so, but not great. CT would certainly show more detail. Earlier you stated a hiatal hernia "can be seen on an xray," which is true in the sense that an obvious one would show up, but not all would show up.Likewise, an ultrasound of the abdomen could in theory show some diaphragm abnormalities, but that's not often what the sonographer and radiologist will be looking at most closely.
I've been thinking the same, and am trying to find out how to know for sure. I'm told that x-rays show the diaphragm, as well as CT scans, but nothing is ever mentioned. I've had an ultrasound of the abdomen as well which I was told would show something if there was a problem, but nothing is coming up.