After setting element.style.fontFamily, how find if that font was applied?

I know that setting the fontFamily only sets it to that actual family if the user’s computer has that font. But even if they don’t, if I set the style, checking “computedStyle” still shows it as the non-existent font family. So how do I find out what font it is actually using?

For example:

Let’s say in firefox on Linux (with no Arial font) i do:

element.style.fontFamily = “Arial”;

but since the computer doesn’t have arial, it defaults to helvetica (or times or whatever). How do I find out that it’s really using helvetica?

Also in #JavaScript _↴