How can diabetic neuropathy be distinguished from b12 deficiency nerve damage?

Mainly Clinical: Your question is a good one as recent reports suggest that about 20% occur together. A few hints to the difference- if you have anosmia (loss of smell), hyperactive reflexes, large greater than small fiber sensory loss, and/or cognitive changes then suspect B12 deficiency. If you have an asymmetric neuropathy, small fiber changes, and/or autonomic dysfunction(bp, sweat...) then think diabetes.

By laboratory tests: The symptoms of a peripheral neuropathy may be very similar, regardless of the cause. Peripheral neuropathy from diabetes, vitamin B12 deficiency, or other reasons usually presents as numbness, tingling, or burning in the feet. Physicians evaluating a patient with those complaints will usually do a battery of lab tests looking for the specific cause.