I spent an hour on google and on the command-line trying to start gucharmap with a given font name at the command line. No dice. (And don't speak to me about man pages....)
It took the source code for "Font Manager" (A rival to Fonty ;) ) and some small hacking to get it to reveal the secret sauce. Here, then, is the way to do it:
gucharmap --font="SOME DAMN FONT NAME, 32"
The "SOME DAMN FONT NAME" can be obtained by:
fc-list : family | grep "SOME"
(IOW: You grep through the results with some small snippet of the name you already know, or just scroll through the output.)
The comma is the secret. The number after that is the point size.