Take the thing and run it under warm water for several minutes to remove as much of the debris as you can.
Then use a good magnifier or, better yet, a loupe. First look for visible cracks and chips on the edges of the facets. Look for out and out cracks as well. Then use the loupe to peer into the stone. If you see small black spots (specks) or white spots (specks) visible in the stone,...they are probably inclusions, as they are called and the stone is likely real.
If you see what looks like bubbles in the stone, it is likely fake. If you see nothing in the stone, It is likely fake unless the setting is white gold or platinum. A perfect diamond would not likely be set in silver.
I don't see any obvious crack or chips around the outer perimeter of the stone so that's good.
Also, if the stone is clear all the way through, that's a good sign, if the back side of the stone, (The pointed part) is coated or "silvered" , it is definitely a clunker. That's done to make a fake stone reflect more light.
Good Luck,
Bill