Fortunately, we have a couple of ways to identify if there is jet-A present in a fuel sample (assuming you’re looking for 100% avgas). Both rely upon that volatility of avgas, and the residue that jet-A leaves behind. First, a thimble-sized drop of the mixture on paper (such as a Post-It note) can show how quickly the avgas evaporates, and leaves a stain behind if there is jet-A present in the sample. This must be observed within a few minutes of the test—after about 30 minutes the paper becomes difficult to “read.” Second, if you use a GATS fuel straining jar, with a clean mesh screen at the top, and strain the sample back out, the avgas should evaporate quickly. Any liquid that remains after a few minutes means the sample is suspect—or you need to clean your jar!