Hi Bill!
A couple of your questions that haven't been answered yet:
How do you do this and how do you know if it's already being done?
When you look at the code for a page, if you see a lot of code within tags, that means that the CSS is on the page rather than in an external file.
You can do the same with JavaScript. If there is a lot of code within the tags in the source code, you know there is JavaScript on the page instead of in an external file.
If it has not been done on a site is it hard to go back and do?
It is very simple to take this code, put it into an external file, then link to it like we do on this particular page's code:
<link type="text/css" rel="stylesheet" href="[/q/moz_nav_assets/stylesheets/production/all.css?0.4.25](view-source:http://www.seomoz.org/q/moz_nav_assets/stylesheets/production/all.css?0.4.25)" />
That is what it looks like when you link to a CSS external file.
If possible though, I'd definitely have someone who knows and understands the code help you out at first. There's nothing more infuriating that making what you think is a small change to a JavaScript file and it end up breaking the whole site.
Good luck! And I hope this helps a bit.