Correct, there is no way to visually identify IBD. IBD is only one of several conditions that can cause what look like neurological type problems with a lot of head flipping and twisting in unusual ways. My understanding is that Inclusion Body Disease is a disease that will cause small inclusion bodies (growths) to appear in the organs including the brain. It's these tiny growths in the brain that cause the neurological symptoms that you see with this disease. However there are many other things that can cause the same symptoms. In spider balls it appears to occur naturally. I've seen it occur in animals that were overheated, this will often disappear with time. I've seen it in snakes that were poisoned (snakes kept on ceder bedding, over use of mite sprays) This will also often get better with time but seems to take longer then with overheating. And I've seen it in snakes that were in the last stages of starvation. In fact any snake that is dying will often appear to lose control of it's bodily movements, that's why we'll often find dead snakes flipped over on their backs.
I'm very sorry for your loss, but there is no way your vet can diagnose IBD with just a physical exam.