You probably need this
Code:
html .redactor_toolbar li a.redactor_btn_custom_sub:before {
content: "\f0c1";
}
Replace the f0c1 code with one from here:
http://fontawesome.io/cheatsheet/
To find any new code, just inspect the code and expand until you can view the actual link and look for the class name there, then replace it like above. Arrows point to the class names: