In most graphical *nix environments that I'm familiar with, if you hold Control and Shift¹ and type U, you can type any Unicode character by its (hex) code point. For instance, if you want an em dash, which is U+2014, hold Control and Shift, and type "U2014".

¹You can release Control and Shift after typing "U", in which case the character will appear after you type a space. Or, you can hold Control and Shift while typing the code point, in which case the character will appear after you release either modifier key.