The food was rubbed onto a piece of filter paper, and left to dry. The filter paper was held up to the light. If the filter paper became translucent (like tracing paper) where the food was, then fat is present.

An alternative is the emulsion test for fats. Put the food that is being tested into a test tube with ethanol. Heat the test tube in warm water, and if it turns cloudy white as shown below then fats are present.

Safety

Ethanol is highly flammable so do not heat it with a naked flame. Heat some water in a beaker to make a water bath, turn off the flame, and heat the ethanol in a boiling tube in the water bath.