You Might Have an XY-Problem
If you’re defining Zapf Chancery and Ralph Smith Formal Script Oblique as math alphabets, you almost certainly want the former to be \mathcal and the latter as \mathscr. (You might only have done your MWE that way as an example, however.) If you find yourself actually using more script alphabets than that in the same paper, you should seriously reconsider your notation.
What you really want to do is load your \mathscr and \mathcal alphabets with mathalpha, as in:
\usepackage[cal=zapfc, scr=rsfso]{mathalpha}
In this case, you would want
\usepackage[scr=esstix]{mathalpha}
which loads the following commands:
\DeclareFontFamily{U}{esstixscr}{\skewchar \font =45}
\DeclareFontShape{U}{esstixscr}{m}{n}{
<-> \mathalfa@scrscaled esstixcal}{}
\DeclareMathAlphabet{\mathscr}{U}{esstixscr}{m}{n}
The remaining command is named \mathchorus, implying that it’s intended to set up TeX Gyre Chorus, a clone of Zapf Chancery. However, it actually selects Computer Modern Sans Serif. This is not a useful thing to do; if you want to use Chorus, use it instead of the default PostScript pzc (which might be different fonts on different systems), and if you want Computer Modern Sans Serif as an alphabet that isn’t the main alphabet, use it as \mathsf.
The Literal Answer
While the font encodings are normally in the package documentation, LaTeX’s New Font Selection Scheme (NFSS) searches for files named [encoding][family].fd. (On OSes such as Linux where it matters, the encoding is lowercase in the filename but uppercase in your document commands.)
So, your examples correspond to the files urfsfo.fd, t1pzc.fd and ot1cmss.fd.
You can therefore search your TeX directory for files matching this naming scheme. For example, on the Ubuntu command line, you could search for all encodings for either the pzc family (Zapf Chancery) or qzc (TeX Gyre Chorus, the clone that actually comes with TeX) with the command:
find /usr/share/texlive/texmf-dist/ -name "*[pq]zc.fd" -print
If you just want to check whether a specific family comes in a specific encoding, you can test whether a specific filename is in TeX’s database with kpsewhich. For example, to confirm that cmr comes in the OML encoding:
kpsewhich omlcmr.fd
Or, all modern file managers have a file search function.
In practice, you can normally try OML first, and if that doesn’t work, OT1, and if that doesn’t work, T1. Nearly all fonts come in OT1 and T1, but mathdesign is one of the few packages that supports OML.
Finally, this is all so much simpler if you can use unicode-math in LuaTeX or XeTeX.
