1. Load 'gucharmap'
2. Select the font ("Ubuntu Mono" in this case)
3. Search (Ctrl+F) for the character itself ('▪')
4. Right click-and-hold over the character
This will show the name of the font that is actually being displayed for that character, rather than what is selected. This is likely to be one of the wide-coverage fonts, such as DejaVu *.
There are probably better ways, but I usually:
1. Load 'gucharmap'
2. Select the font ("Ubuntu Mono" in this case)
3. Search (Ctrl+F) for the character itself ('▪')
4. Right click-and-hold over the character
This will show the name of the font that is actually being displayed for that character, rather than what is selected. This is likely to be one of the wide-coverage fonts, such as DejaVu *.