Maria, Celaine, and Elenei weren't sure how they should test if it was real or not. They could try becoming the Master of the Core, but that's easier said than done even if they had the real thing. The main problem with this is even if they do become the 'master' of the Core, who's to say they didn't just waste all of their time becoming the 'master' of a fake core?
The same issue applies to the other possible solutions as well. They could try and destroy it, but if it was fake then they would have just wasted much of their time and energy on something that brought them no benefit.
But, they are only worried about these things because they are short on time. If they had more time to spare, then things would have played out differently.
