From the book: Symplectic Geometry by Ana Cannas da Silva, page 19. The following statement is claimed, but I don't full understand it, neither know how to prove it on my own.
Two functions generate the same lagrangian submanifold if and only if they differ by a locally constant function.
E.g. "locally constant" sounds fancy but it really means, constant on each connected component of the manifold they're defined on; usually we have just one connected component and mean a constant function.
– Elizabeth S. Q. Goodman Mar 28 '17 at 16:17