How to write ≙ in latex

Use \overset from amsmath:

\documentclass{article}
\usepackage{amsmath}
\newcommand{\estimates}{\overset{\scriptscriptstyle\wedge}{=}}

\begin{document}

$A\estimates B$

\end{document}

It won't work in sub/superscripts, but I don't think you need the symbol there.

A different realization is with

\newcommand{\estimates}{\mathrel{\hat{=}}}

that would produce

With the package fontspec, you may call each symbol by its unicode. In this case, the command would be $\symbol{"2259}$. You just have to look for fonts, which are giving you this symbol.

Here are 4 examples:

% arara: lualatex

\documentclass{article}
\usepackage{unicode-math}
\usepackage{booktabs}
\usepackage{fontspec}

\begin{document}
\begin{tabular}{ll}
\toprule
Example & Font\\ \midrule
$a\wedgeq b$ & Latin Modern Math\\
\setmathfont{xits-math}$a\wedgeq b$ & XITS Math\\
\setmathfont{asana-math}$a\wedgeq b$ & Asana Math\\
\setmathfont{texgyrepagella-math}$a\wedgeq b$ & TeX Gyre Pagella Math\\
\bottomrule
\end{tabular}
\end{document}

You can use STIX fonts if you don't mind.

\documentclass{article}

\usepackage{stix} % (Times style) Main fonts of the document will be changed

\begin{document}

$a \wedgeq b$

\end{document}