If $V$ is a vector space with finite dimension and for some symmetric bilinear form $f: V \times V \rightarrow \mathbb{R}$, how to I show that $f$ defines an inner product iff the unique real canonical form of $f$ is $I\_n$.

So I know that we can represent the bilinear form as a quadratic form and from sylvesters law of inertia we have that there must be exactly one canonical (Thus proving uniqueness). However, for this how do I start showing that the basis elements are all orthogonal with full rank?

Any help would be greatly appreciated!