One way to determine the version of HTML5 SDK used on a page is to look at the HTTP requests after clearing your local browser cache. You can use Firebug or Charles for this process.
For the video viewer, you can see the SDK version by looking at /s7viewers/html5/js/VideoViewer.js.
Look for the this.sdkBasePath. For example:
this.sdkBasePath = '/s7sdk/2.4/';
For the video viewer, you can see the viewer type # and version by looking forthis.s7params.setViewer.