I was set up at a show selling my gold nuggets next to a person who was a science teacher and he taught me this fun way to test gold.
Take a small amount of liquid makeup foundation and apply it to the top of your hand. Let it dry for a couple of minutes. Take an item you know is gold and rub it across the makeup.
A dark line will show up on the makeup! Take an item you know is not gold like brass and do the same thing. No dark line. It cannot tell you if it is 14 karat or 10 karat but it is a easy way to test for gold when say you are out at a flea market or garage sale.