To check if website is using Flash, you need to check for either embed tag or object tag, for embed tag you can see the application type which should be shockwave , and for object you can see the CLSID value which is same on windows for sure. to see if Flash plugin has loaded.

Adobe Flash (formerly Macromedia Flash and FutureSplash) is a multimedia software platform used for production of animations, rich internet applications, desktop applications, mobile apps, mobile games, and embedded web browser video players.