How can I determine which fallback font is being used for a given character?

I'm using LibreOffice (7.4.1.2) on Devuan (Daedalus). I've set the right-to-left language group font to some font, let's say it's font family Foo in size 12, regular; and I type a few glyphs in Hebrew or in Arabic. Problem is, this Foo font has no glyphs of Hebrew or Arabic characters. So, I do see some glyphs appearing... my question is: How can I determine which fallback font these glyphs are taken from? Any reasonable method would do, either using a GUI tool or the command-line.