We consider the expansion of the square of a complete homogeneous function $h_λ$, or of an elementary symmetric function $e_λ$, in the basis of Schur functions. This square also decomposes into two plethysms, $s_2[h_λ]$ and $s_{11}[h_λ]$ (resp. $s_2[h_λ]$ and $s_{11}[h_λ]$), which are called its symmetric and anti-symmetric parts, respectively. We define a sign statistic on the set of tableaux that index the Schur functions appearing in the square of those symmetric functions. This sign statistic allows to determine to which plethysm each Schur function contributes. We use mainly combinatorial tools on tableaux (product on tableau and RSK) and basic manipulations on plethysm and symmetric functions.