Without x-ray eyes, this would not be possible. However, there are supposedly clinical indications that should help us work it out. They are not very reliable.
However, osteopathic treatment is often aimed not at the part in trouble, but the surrounding areas and the environment which the disc inhabits. By ensuring that the tissues in question get the nutrition they require and a state of mechanical ease, fewer adverse forces focussed at them, then they do usually recover on their own - that's the theory.
If this approach to treatment is applied, then it shouldn't make things worse, and in practice it is often effective. The reality is that many problems initially diagnosed as a disc on the clinical picture alone can be made worse by treatment, for reasons I can't go into in a single post. For the same reason, many problems do hide their true nature. Then, by the time the patient gets to a scanner, it really is a disc problem.
We could screen everybody by MRI first, but then the costs and waiting times would exclude many people from help, and almost all from early help. Much better, in my view, to be mindful of all possibilities behind the presentation, and then treat the patient in a way that will not make things worse if it is a disc. What that means in practice, of course, could fill up a library.
Nevertheless, the chain of errors involved does not apply specifically to Osteopaths. We see this arising from massage, specific manipulation, prescribed muscle relaxants etc etc. Any profession has the tools to make things either better or worse. What this highlights yet again is going to a practitioner with a good reputation - whatever profession you look into, this is the best you can do.