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

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.
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.