  Let $f$ be a homogeneous polynomial of even degree $d$. We study the
decompositions $f=\sum_{i=1}^r f_i^2$ where $\mathrm{deg}\ f_i=d/2$. The
minimal number of summands $r$ is called the $2$-rank of $f$, so that the
polynomials having $2$-rank equal to $1$ are exactly the squares. Such
decompositions are never unique and they are divided into
$\mathrm{O}(r)$-orbits, the problem becomes counting how many different
$\mathrm{O}(r)$-orbits of decomposition exist. We say that $f$ is
$\mathrm{O}(r)$-identifiable if there is a unique $\mathrm{O}(r)$-orbit. We
give sufficient conditions for generic and specific
$\mathrm{O}(r)$-identifiablity. Moreover, we show the generic
$\mathrm{O}(r)$-identifiability of ternary forms.
