gem sources confusion

Now that Rails and many other gem/plugins have moved to github, there
needs to be a better way to know where to get things from. The first
thing I did was add github as a source (sudo gem sources -a
http://gems.github.com/). How do I know where the gem came from once I
install it? For example, Capistrano 2.4 seems to be in both places.
Which one do I get when I install? If installed from rubyforge, but
github gets an update, will gem update pick it up?

Do I need to go through all my gems and re-install if there is a later
version on github?

Plugins are even more fun.

Any suggested courses of action for managing my gems in plugins?