I have a keybinding I just made that essentially will go to the end of a
line, insert a semicolon, return, and auto-indent.

I'd like to only add the semicolon if it doesn't already exist.

In other words, go to the end of the line, if a semicolon is there, return.
If a semicolon is not there, add one, then return.

I'm brand spanking new to emacs, and I'm sure this is a task that others
have resolved, but I'm having trouble googling for a solution.

Thanks,
Steve