The actual JS source is minified which makes it almost impossible to read (but if someone has the time to pick over it and post an 'autopsy of a stickman' blog post I'm sure the karma will flow readily!).
Often, a quick right-click will be enough to expose flash; JS/canvas/html5 will give you the normal context menu, flash normally produces its own unhelpful context menu. That's a good indicator.
If you have to go back to the source, what you're looking for in determining if something is flash or not is an <embed> or <object> in the main HTML. That's not always a sure indicator - it could be being added in JS. Likewise the presence of a <canvas> element is a good indicator that it's JS.
In this case, the huge number of .js includes is the biggest hint - 18 <script src=...> elements, including telltale file names like 'stickman.js', 'animation.js', etc.
This really makes me want to dive into some of this new web hotness :)