I suspect you want the question mark to point along the line at the end. You could use the sloped to align the question mark perpendicular to the line and then rotate it 90 degrees to make it parallell. Also you'd need to reposition the question mark as it would have it's anchor point in the middle.
\documentclass[margin=0.5mm]{standalone}
\usepackage{tikz}
\begin{document}
% Get height of question mark
\newbox\qbox
\setbox\qbox\hbox{?}
\edef\qmarkHeight{\the\ht\qbox}
\begin{tikzpicture}
\node[draw=white] (1) at (4,0) {AA};
\node[draw=white] (2) at (0,4) {};
\node[above left] at (2) {BB};
\draw[black, arrows={|-}] (1) -- (2) node[sloped, at end, rotate=90, yshift=\qmarkHeight/2]{?};
\end{tikzpicture}
\end{document}
Which produces this "tip": 
Additionally you can make it a style (\draw[questionmark]...) using decorations, as per this answer:
\documentclass[margin=0.5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}
% Get height of question mark
\newbox\qbox
\setbox\qbox\hbox{?}
\edef\qmarkHeight{\the\ht\qbox}
\begin{tikzpicture}
\tikzset{questionmark/.style={
postaction={
decorate,decoration={
markings,mark=at position 1 with {
\node[at end, rotate=-90, yshift=\qmarkHeight/2]{?};
}
}
}
}
}
\node[draw=white] (1) at (4,0) {AA};
\node[draw=white] (2) at (0,4) {};
\node[above left] at (2) {BB};
\draw[black, arrows={|-}, questionmark] (1) -- (2);
\end{tikzpicture}
\end{document}
Which produces an (almost) identical result.