Eclipse provides a single click and download for the support library for an android project, and it adds it to the libs/ folder as “android-support-v4.jar”
The above jar has no reference to revision number (currently r10), so surely (unless I use maven) how are other people keeping up to date with the support library revisions?
I agree with @yorkw, the SDK manager tells you which version is currently installed. Also, using the android tools -> add support library will in-fact install a newer version if available, you will see that later in the SDK manager.
If what you are asking is how you know a new version is available so you can update? than the answer is definitely SDK manager, as yorkw said.