The easiest way to tell if mineral oil is bad is by using a smell test. Mineral oil in its pure form has no odour—it should be totally odorless. If you detect any kind of smell from the mineral oil then it has been contaminated with something, which means it has gone bad and should not be used. Additionally, when checking for signs that your mineral oil has gone bad, check for discoloration or cloudy appearance in the bottle—both are indications that the product is expired and needs to be discarded.
Another way to check if your mineral oil has gone bad is by conducting a “spot” test on a white cloth or paper towel. Place some of the liquid on the white surface and look for changes over time; if any stains appear or remain then this could indicate contamination, thus making it unfit for use.
Ultimately, once you have completed a visual inspection and confirmed there are no visible signs of contamination or discoloration, try using your senses (smell/taste) one last time before applying to makeup brushes or skin products—this can help prevent potential harm due to long-term exposure from spoiled products.