Delta-equal to symbol

How about \triangleq from the AMS fonts? Loading \usepackage{amssymb} will make the command available

EDITED to ensure rendering of the = in math mode. For more flexibility in the vertical placement, than the \overset (which requires amsmath package), you could try

\documentclass{article}
\usepackage{stackengine}
\def\delequal{\mathrel{\ensurestackMath{\stackon[1pt]{=}{\scriptstyle\Delta}}}}
\begin{document}
$a \delequal b$
\end{document}

The optional argument to \stackon specifies the vertical gap between symbols. If you wanted the delta smaller, you could add \scriptscriptstyle in place of the \scriptstyle:

For comparison, here is $a \overset{\Delta}{=} b$. Note the vertical offset is significantly larger than what I proposed.