How can you tell if a substance contains fat?

Rub two samples of food nicely on a clean sheet of paper. Let the paper dry for some time. Then put the paper in front of a bright light. The paper on which food containing fat was rubbed, would have a little glow(it looks like the paper is a little wet). The other paper, on which the food containing no fat was rubbed, would not look like that.

Note: The paper must not be too thick.