How can I tell if Flash is loaded on a website?
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. i am not sure how to do that.. but to see if Flash file is loaded from network you can listen to 'readyState' event of the tag and wait till it becomes 4.
One of the best ways to see the generated html code is using the "Web Developers Toolbar" in firefox or chrome (it's a plugin). This way you can see if there is a object included, and possibly see if it is a .swf or if it says flash content.https://chrispederick.com/work/web-developer/
Are you looking for how to detect at the client if flash is being used? If so, flash has the ability to interact with javascript. Here is a website that outlines this
https://www.permadi.com/tutorial/flashjscommand/
You could have the flash plugin set some javascript variable on startup which can be used to determine if flash is running or not.