You can test items that you suspect to be gold filled in much the same way that you would test any other type of gold using the streak testing method. The biggest thing that you want to be on the look out for is for a fading or disappearing golden shine as you make streaks on the testing stone.
Take the item you suspect of being gold filled and scratch a sharp corner or edge of the item on the stone as you can get through the surface faster there than on a flat area. As you do this, you will notice that the metal leaves a mark on the testing stone. Press firmly on the testing stone to leave a thick and visible deposit that you can test accurately that is about 1 inch long.
Repeat Step 1 and make a second mark next to the first one with your item scratching with the same section you did before.
Repeat Step 1 one last time to make a third mark on the testing stone just next to the second mark, still scratching with the same section you did at the beginning. When testing most gold filled items, one streak is all that you need to see that the metal streak will not hold up to acid testing but it is always best to be thorough in your testing.
Apply a streak of you 14 karat acid across all three streaks of unknown gold and observe the reactions. If the second or third streak fades out or significantly changes color the piece is heavy gold filled, if they fade away completely you can be sure that there is no gold content in the metal.