= Solution
A <Hermitian form> is conjugate-linear in its first argument, linear in its second, and satisfies $\phi(w,v)=\overline{\phi(v,w)}$. For $H=\phi-\psi$, the hypothesis says $H(v,v)=0$. The complex <polarization identity>
$$
4H(v,w)=\sum_{k=0}^3i^kH(v+i^kw,v+i^kw)
$$
therefore gives $H(v,w)=0$ for all $v,w$.
The displayed matrix is Hermitian. By <Sylvester's criterion>, its leading principal minors are
$$
4>0,
\qquad \det A=12-(2i)(-2i)=8>0.
$$
Hence the form is
$$
\boxed{\text{positive definite}.}
$$
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