Can vit B12 deficiency cause permanent nerve damage? If yes, how to know? Is there a test or a procedure to see visually the nerve damage?

Check Homocysteine. Elevated homocysteine levels are a good marker for nerve damage due to B12 and Folic Acid deficiency. Even with normal levels of these two chemicals your body may not be able to use and you need to take the methyl- varieties and can lead to functional deficiency leading to neuropathy. There is test that looks at the small fibers that get affected by these conditions and diabetes. Ask your doc.