No it's not hard to say what's Constitutional. It's just hard to say that the Supreme Court will uphold the Constitution. We can disagree on what we each think it means, but it's not clear that the Supreme Court really knows either.
And [1] is a sort of self-authority granted.
[1] Marbury v. Madison, 5 U.S. (1 Cranch) 137 (1803)
Determining what a law means is an essential part of resolving controversies under that law, and Art. III expressly extends the judicial authority of the United States, vested in the Supreme Court and subordinate courts, to, among other things, all cases and controversies arising under the Constitution. "Judicial review" is simply resolving cases and controversies arising under the Constitution, and could not be more express of a power.
You're contradicting yourself. The idea of people disagreeing on the meaning of various bits of the Constitution is the very definition of it being hard to say what's Constitutional.
And [1] is a sort of self-authority granted.
[1] Marbury v. Madison, 5 U.S. (1 Cranch) 137 (1803)