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In the scenario involving a 53-year-old woman with symptoms suggestive of Wernicke's encephalopathy, the most likely diagnostic test is the response to thiamine administration. Wernicke's encephalopathy is primarily caused by thiamine (vitamin B1) deficiency, which is often seen in individuals with chronic alcoholism or those with malnutrition.

When thiamine is administered to a patient exhibiting the classic triad of symptoms—ophthalmoplegia, ataxia, and confusion—an improvement or resolution of these symptoms can be observed, serving as a diagnostic indicator. This response is particularly telling because it not only confirms the diagnosis but also acts as a therapeutic intervention.

The other options do not provide the same level of diagnostic clarity for Wernicke's encephalopathy. For instance, administration of edrophonium is typically used in diagnosing myasthenia gravis rather than in identifying vitamin deficiencies. Noncontrast head CT might be useful for ruling out other conditions but is not specific for diagnosing Wernicke's. Lastly, serum magnesium levels, while they may be relevant in the clinical context of a patient presenting with neurological symptoms, do not specifically diagnose Wernicke's encephalopathy. Thus, observing