you can. and you can not. it depends on the browser and the userīs settings.
www.macromedia.com has a script for that, but i found better ones on the web.
they work like this:
- check browser AND OS (eg. on IE/MacOS there is no VBScript! and for MacOS, Netscape and Flash<=5 the FSCommand interface does not work - and so on)
-- IE/Windows: create object using vbscript, if it fails there is no flash
-- IE/Mac - output the flash tags with auto-download option.
-- NS: check the navigator.plugins array for occurences of "flash"
-- if none of these succeeds, output a 1x1 flash movie that only forwards the user to another page (the entry to the pages with flash) and use <meta refresh> to load the non-flash page after some seconds.
you can see, if javascript is disabled, you cannot detect flash. the only reliable way is to let the user choose...

this is why i donīt use flash for web-pages but only decoration and nice menus
(you can supply <noscript> and <noembed> tags to display another menu instead if there is no flash, but i donīt consider this "plugin detection")
and the same deal with MF QuickTime!