The default publish settings for HTML in Flash have simple plug-in detection for Explorer and Netscape. Script can be copied from this to your HTML page to detect the plug-in. This will cause Flash player to either auto-install or it will take your user to the Macromedia site to download Flash Player.
In Flash Method
The easiest way to do Plug-in detection is
in the File>Publish Settings, HTML area. There select the Detect Flash Version
check box and click on the settings tab (at the top of the HTML tabbed area).
You will be given the options for creating an HTML file that will serve up
a link to Macromedia if Flash cannot be found.
It creates 3 files: Detection HTML, Content HTML, and Alternate HTML as well as a Macromedia Gif icon. The content is your html that points to your Flash SWF. The Detection is your first file that you link to (you can rename this index.html if it is your first page in a folder online). The alternate.html serves up a page that will link to Flash to get the Flash Plug in.
Other Methods
Another method for detection is with Javascript
(Netscape) and Visual Basic (Explorer). This is more complicated to produce,
but the script can be reused once the file is created.
Another method involves making a "Swiffer" movie. This is a small Flash movie which is not seen by the user. It sends the user to a URL with Get URL for your movie if the plug-in exists. Otherwise, if the player is not installed, the movie won't play and a <META> tag in the <HEAD> of the HTML that houses the swiffer will send the user to a screen telling them to download the Flash player.
Download some alternate detection sample files