This one's pretty easy to track yourself, actually. Crack open the dev console on your browser and see if there's any traffic while you type or move your mouse. Many websites will also track your mouse movement and aggregate it all to form heatmaps and see how people are interacting with the UI.

The solution is same as it ever was: disable JS by default, and if you have to enable it, use something like the uMatrix extension to selectively kill scripts.