I think that only a MRI scan will reveal disk damage. With William we skipped the x-ray stage altogether and I think that was because my vet thought William might have disk damage and the scan would give the definitive answer.
Like Lisa has said, it's probably best to check if your insurance will cover it. My insurer would still have paid up even if the scan showed nothing because my vet had made the referral to the specialist. It was me who asked for the referral but the letter from my vet didn't say it was at my request, simply that he (vet) wanted to refer William. I sometimes wonder if my insurer might have queried it if they knew I was the one pushing for the referral.