compare a value from \newcounter to value from \newcount

The name of the internal counter is \c@testB. And \value will work if you remove the backslash.

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[german]{babel}

\begin{document}

\newcount\testA
\testA=1

\newcounter{testB}
\setcounter{testB}{1}

\makeatletter
\ifnum\testA=\c@testB
TRUE
\else
FALSE
\fi
\makeatother


\ifnum\testA=\value{testB}
TRUE
\else
FALSE
\fi
\end{document}

The solution is to use

\ifnum\testA=\value{testB}