To date, Google has confirmed each Panda update and that's the main signal I've been looking for. That confirmation has always been coupled with a decent buzz about ranking losses and gains in the webmaster discussions. When a Panda Update is confirmed, I record it here: [webmasterworld.com...]
Several companies that specialize in data about ranking changes (notably Sistrix and SearchMetrics) usually publish their own analysis of winners and losers at the time of a Panda update. It is not an easy job to identify a Panda change without massive amounts of data - a very specialized endeavor - so I follow their articles.
To differentiate the other significant but NON-Panda updates, I watch our own monthly thread - this very one.
In addition, Google has begun to use their Inside Search blog once a month to publis an article about all the changes in the previous month. The first such article was [insidesearch.blogspot.com...] and the second should be coming early in the new year.