Sorry I ment bulging disk. please forgive me.

You are right in saying that you can not see the disk on Xray. However, even though Xrays are not used to see soft tissue if there is enough inflammation you can see the disk.
To see a disk you need an MRI. The MRI brings up a whole different issue as far as its reliability. I have little to no interest in MRI because it is a non-weight bearing study. Due to the fact that most of the MRI studies done are in a supine (face up) position and within minutes the curve will change. Depending on how long the MRI takes creep (stretching of ligaments) will set in. The lordosis in the lumbar and cervical spine begin to flatten out the longer you are laying down giving the appearence of bulging disk(s).
I am VERY excited about the future of MRI in a weight bearing postion. The one that I have seen is done in a seated position which you have to take into account the lowest lumbar which will be altered by the sitting position so this will have to be taken into account. Very few places offer this service right now and the places that do offer it do it at about 5 times the cost or more.