Can you spot any inclusions after looking up close through magnifying lens? Also, do you have UV light source? If the diamond has florescence, it will glow blue when it is exposed to UV light. Not 100% of course, but fake diamonds usually will not have any inclusions (impurities) and will not have florescence.
Your best way to determine is to take the stone to a jewelry store. I would not rely on home methods bc there are some good fakes, plus many jewelers have testers to also ck to see if the stone is a moissanite. You need to have it tested with both testers.
Clean it and see if you can see the writing flash past if you place it table down on a page of print.
Most fakes are easy to see thru.
Diamonds are not.