as a maintainer of fifty-something AUR packages I found it difficult to keep track of new upstream versions. I would subscribe to various RSS feeds to keep track of some.
Recently, I sat down and wrote a tool "aur-out-of-date" which, given a username or a list of packages, checks upstream for newer versions. It does so by inspecting the upstream URL and/or source URL, and calling a suitable API (github.com, registry.npmjs.org, pypi.python.org, cpan.org for now) to determine the latest released version. Out-of-date packages are output in red color on the command line.
I'm happy to hear you feedback :) Simon