Nerve Damage Back How Do They Figure Out If You Have A Pinched Nerve? MRI? Or Just Nerve Damage Period Specifically In The Back?

How do they figure out if you have a pinched nerve? MRI? or just nerve damage period specifically in the back? - nerve damage back

Directly below the right shoulder blade. there was no pain for 3 + year, and when I sleep in certain positions that bother me and is very unpleasant. What is it?