In other words, can I know if somebody uses script or CSS extension tools?
For example, he aligned things differently, placed
display: none; on several elements, used jQuery's
.hasClass to add or remove divs?
I don't think there's any one-shot, easy, "Hey, they modified the page" way. You'd have to have a record of exactly what elements you were expecting to be on the page, and their effective CSS properties, and then check all of those at runtime against the actual DOM and the effective CSS properties of the elements in the DOM.
It's going to be prohibitively complicated and hard to maintain.