The quickest way to tell if a drink has alcohol in it is simply to smell it, or take a quick sip. However, if you want to use more objective methods to test the contents of a drink, there are more decisive ways to determine if a drink contains alcohol.
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Use a Drink Alcohol Test Strip
Purchase a test strip that is detects alcohol in liquids. Remove the test strip from the packaging and dip it into the drink. Pull it out immediately, right after the strip gets wet. These usually take just a couple of minutes to show results. Usually, you'll need to consult a color chart to translate the results into percentage of alcohol detected.
Use a Saliva Alcohol Test Strip
Saliva alcohol test strips detect the presence of alcohol in a drink already consumed. These tests are used to assess a person's blood alcohol level, and advise that you should wait at least five minutes after drinking or smoking to use, since they are extremely sensitive. Using these immediately after a drink has been consumed will render the results inaccurate; however, they will prove absolutely whether a drink has alcohol in it. These strips are placed under the tongue for about 20 seconds, until they are fully saturated with saliva, and then removed. Within about a minute, the results are displayed via color, which translates to an approximate blood alcohol level.
Use a Hydrometer
A hydrometer -- which is a small tube-shaped tool that contains either lead or mercury -- tests the density of a liquid relative to the density of water. Alcohol has a lower density than water; sugar has a higher density than water. Hydrometers are generally used by people who produce alcoholic drinks at home, such as wine and mead, and want to know approximately how much alcohol is present. Pour the drink into a test cylinder, leaving room at the top for liquid displacement, then insert the hydrometer. Use an online calculator to translate your Specific Gravity, or SG, reading into percentage of alcohol. Be sure you are using a hydrometer specific for your purpose, as many hydrometers measure specific ranges of alcohol and sugar levels.
Try putting it in the freezer. If a drink consists of 20 to 25% alcohol, it won't freeze. 24-proof liquor freezes at around 20 degrees Fahrenheit, and 64 proof liquor freezes at around -10 Fahrenheit. Don't test unknown liquid in glass in the freezer, though. Frozen water expands, and could shatter the glass.