I'm not sure why you're asking...but here's a semi-useless answer.
The short of it? You've got to diff what you have with what's out
there (or know where you got it).
Unfortunately it doesn't look like the plugin has a version or
license file in it by default. So it depends on how you installed it.
Recently it moved to the ubiquitous github, there are several 'new'
versions up there. So if you either used git or downloaded directly
from there, you've got one of those 'recent' versions.
If however you did what I did and ran script/plugin install <URL>
then you probably got the latest svn version up on the technoweenie
site.You can figure out what version this is by going over to <URL>
with a browser and recording the version info (although the repository
contains "everything" so the version info changes even when there are
no changes to restful_authentication itself).
I think most everyone is just maintaining their own copy which is why
git is going to be more useful than svn (sure wish it'd been mercurial
instead...oh well, nothing quite like having 3 version control systems