The usual test for lipids involves a physical change using a substance called Sudan III. Sudan III is a nonpolar organic solid which dissolves in other nonpolar substances, but will not dissolve in polar substances such as water.


>> Polar and Non Polar <<



The above molecule shows the structure of Sudan III, the test itself is quite straightforward, if there is a lipid (fat or oil) present the Sudan III will stain the lipid red, completely dissolving in the nonpolar solvent whereas in a polar solvent such as water, the nonpolar Sudan III will simply refuse to dissolve.



The above picture shows Sudan III dissolved in a 100% nonpolar substance, the reason that there are some lumps of the chemical hanging around is simply that too much Sudan III was added and it could not dissolve. in the polar substance it is quite clear that the dye refuses to dissolve. 


In an actual practical application there would be combination of polar solvent, and if present nonpolar lipids. Small droplets of red dyed fat will be present if the test is positive, but more often than not, in a test tube application the nonpolar lipid would rise to the surface of the polar solvent and would appear to be dyed red so what you would in fact get in a positive result would be a separation into two layers, the top of which would be red because of the stain.