I'm trying to determine if a browser that my content is being pushed to supports flash/silverlight. That way, if there is no support on the browser, I can interchange the flash/silverlight components with HTML/AJAX components.
This is probably going to depend upon exactly what you're doing. Whatever method you're using to embed the Flash or Silverlight probably already provides a way to do this. For example, here's an article on how SWFObject handles this.
I recommend the jQuery browser plugin detection plugin.