How to type the symbol of vargha and delaney a effect size?

The comments are perfect! But if a day you would to use two accents my suggestion is to adopt accents package.

Here a MWE with a screenshot.

documentclass[12pt]{article}
usepackage{accents}

begin{document}
$hat{A}_{12}$, $hat{accentset{circ}{A}}_{12}$, $hat{accentset{diamond}{A}}_{12}$
end{document}