Methanol is an alcohol much like ethanol, which is the active ingredient in alcoholic beverages. Methanol provides the same buzz as ethanol, and occurs naturally at low levels in fermented drinks but is far more toxic than ethanol, to the point that it can cause severe illness and death. Commercial producers of alcohol have specialized methods of removing methanol from their products, but home and hobby brewers tend not to have the technology at their disposal to easily remove the substance from their brews. At the same time, illegal breweries will sometimes use methanol as a cheap substitute for ethanol. Luckily, there are ways to test for the presence of methanol in an alcoholic beverage.
TL;DR (Too Long; Didn't Read)
Though similar to ethanol, and capable of providing the same buzz, methanol is a toxic substance and should not be consumed. In trace amounts, as found in certain fermented drinks, it is not harmful, but in larger quantities it can be deadly. Alcoholic drinks containing methanol will sometimes have a pungent odor and will produce a yellow flame when lit on fire. For a safer test, you can apply sodium dichromate to a sample of the beverage.
Though methanol is an alcohol similar to ethanol, it is incredibly dangerous in large quantities. While methanol does form in small amounts during fermentation and is fine to consume in things like commercially produced wine or beer, the concentration you find in things like home brewed gin, rum and other spirits can poison you. Unlike ethanol, when consumed, methanol in the human body is converted into formic acid. The same substance found in ant venom. A buildup of formic acid resulting from this can cause circulation problems, liver damage and a number of other symptoms up to and including nerve damage, permanent blindness and kidney failure.
If you suspect that an alcoholic beverage may contain a dangerous amount of methanol, there are a number of quick and crude tests you can perform. The easiest is to smell the beverage: If it has a strong, unpleasant chemical odor, the beverage may not be safe to consume. However, because not all methanol-tainted beverages produce this smell, it is also possible to test with flame. If a sample of the beverage is lit on fire, and the fire burns yellow rather than blue, the drink is not safe to consume.
Testing alcohol by scent or by flame are neither guaranteed nor safe methods, however, to test for the presence of methanol more effectively, you can apply sodium dichromate to a sample of the beverage. To do so, mix 8 mL of a sodium dichromate solution with 4 mL of sulfuric acid. Swirl gently to mix, then add 10 drops of the mixed solution to a test tube or other small container containing the alcohol. Swirl this container gently a few times, then waft the air from the mouth of the container towards your nose by fanning the air toward you with a hand, with the container placed roughly 8-12 inches from your face. Take note of the scent: If it is pungent and irritating, methanol is present in the alcohol. If the scent is dominating and fruity, only ethanol is present, and the beverage is safe.