You buy test strips that will give you positive or negative results. You dip one in fluid and anxiously await the results. Time's up. The result is positive--your D+Caf Caffeine Test Strip has confirmed that your server didn't give you the decaffeinated coffee you ordered. 

D+Caf Caffeine Test Strips are a great innovation for those who are monitoring their caffeine intake. You dip the D+Caf strip into your coffee or tea--before adding milk or any sweetener--for five seconds. Once finished, you pull the strip out and it shows you how much caffeine is in your drink.

The D+Caf test strips, according to the product description, are 98% accurate. You can also use them for determining caffeine in cold drinks.