What would be the most sensible way to determine if tingling/pins and needles sensations are being caused by a b vitamin deficiency/toxicity?

Blood work: Blood work can help to see if levels are low plus you can have a nerve conduction study done to see if that is the problem However that test does hurt some. Taking a course of B vitamins and seeing if that helps is the best because you probably need the vitamins anyway

Take the vitamin...: Depends on whether you want to see a doctor or not. If not, then I would take a B-complex vitamin supplement for a month or two and see if it helped. A better way would be for your to see a neurologist and undergo evaluation for neuropathy.

Lab: get a serum B12 and folate (folic acid) level and I would also recommend a fasting blood sugar. Your symtoms require a comprehensive examination and thorough blood and urine evaluation.

Check level: Vitamin levels can be measured with laboratory testings. Abnormal levels with symptoms most probably related to that cause.

Obvious: The most sensible way is to see your doctor and for him/her do a careful and thorough history and physical examination and then generate a differential diagnosis. Why are you so fixated on a vitamin? There are all kinds of different possible causes for your symptoms. Begin at the beginning; don't try to guess at the end when you have absolutely no data on which to base an answer.

See a DR: Need evaluation of entire history and exam. Blood can be checked for Vit B12. Anxiety with hyperventilation can cause such symptoms. Do not assume to know the answer. Do you have a normal diet? How much alcohol? Prior surgery? Meds or supplements?