I had to find this out recently and it’s usually qui0 te easy on a modern browser such as Chrome (who just went stable with version 12) or Firefox 3 or 4, But try finding this in Safari.
Fear not A quick google found this really handy page which not only shows you the version that the browser is using but the current stable version you can install. Here is the link http://flashbuilder.eu/flash-player-version.html
