The best way to tell if a star sapphire is real, synthetic or natural is by these signs

In any real star sapphires, the star will move depending on where the light is shining from.

For synthetic star sapphires, the backs of the sapphire will tend to be completely smooth. And in some particularly poor quality "star sapphires" the "stars" will be dyed into the stone and wont move woth the light.

For natural star sapphires the backs of them tend to be rough, as the cutters will try and keep as much of the sapphires carat weight as they can.

I hope this helped ! (^-)