This question is not necessarily solvable using the command line (as you seem to want to do). No forking happens, the player runs inside the process that handles this exact webpage, making it impossible to check through
However, the solution is accessible by the means of the website source code. The following solution may work:
$ curl http://example.com/ | grep '<video'
Note that the website is likely to be using scripts to place the player and you need to use Firebug or the WebKit Inspector to access live website code and search for the
video tag there.
And for YouTube, alternate solution is looking for
"html5": true in the source code, but HTML5 on YouTube would require authentication, which is hard to do with