I think glucocorticoids have caused my muscle cells to die. how can i prove this?

No need to: Glucocorticoids always make skeletal muscle cells shrink. My hope is that when you no longer need to take them, you'll be able to return to your former level of conditioning. A neurologist can tell you more about actual steroid myopathies, which may involve permanent damage. Good luck.

Nothing to prove: It's certainly possible to lose muscle particularly after long term high dose steroids. If you're now off of them, exercise can return some muscle mass, but it's very slow.