Herein are disclosed methods and devices for optically monitoring multiple parameters of an oil sample. In one embodiment, the methods and devices can be used for determining the quality of cooking or flying oil in terms of the free fatty acid content and total polar compound content of the oil. The methods use an optical absorbtive/reflective property in evaluating the free fatty acid content, and use optical fluorescence in evaluating the total polar compound content, with both measurements using a single sampling substrate and a single measuring device.