Determine whether the macro/function is in subscript, in superscript, or neither

Question

I would like to provide different formatting for an xparse command, depending on whether the command is expanded in superscript, in subscript, or neither.

That is, I would like to know inside the command

\NewDocumentCommand{\command}{}{%
    % here
}

whether it is being invoked like

{}^{\command}

{}_{\command}

or

\command

A question like this has already been asked: Can I detect whether my macro is being used inside a subscript or superscript?. I would like to find a direct solution I could later expand upon, provided I'm working with latex3 and xetex, if either is of relevance here.

If there is no direct means provided by the kernel, I believe a comparison with the size of the box (?) in which \command is expanded may be a good proxy.

---

Thank to the comments some misconceptions dissipated and I've come up with an approach to "dimension measurement" referred to elsewhere.

In the example provided in the comments, I now have replaced the fixed-height \big| strut (vphantom) with a strut constructed as follows.

\savebox{\testbox}{\ensuremath{#1}}%
% \settodepth{\dimen0}{\testbox}%
\settoheight{\dimen1}{\testbox}%
% \addtolength{\dimen1}{\dimen0}% Not universally appropriate.
\addtolength{\dimen1}{0.375 \dimen1}%
\rule{0pt}{\dimen1}%

where \newsavebox{\testbox} is invoked outside of the macro.

This, however, does not properly set the height of superscript or subscript arguments, e.g.,

$\settoheight{\dimen0}{k}$ \the\dimen0  == ${}^{\settoheight{\dimen0}{k}}$  \the\dimen0

are identical.

It is furthermore still lacking the self-referential property desired.