If you're in the possession of a 10x loupe, you can easily check for groves. A real yellow sapphire will be completely smooth, while a phony stone will have noticeable imperfections. Under 10x magnification, you might notice grooves similar to that on a vinyl record. If that's the case, the sapphire is synthetic. When you are looking at a yellow sapphire, the best way to know if it is working is to look at the color. If the sapphire is a light yellow, it is not working. If the sapphire is a darker yellow, it is working.