I think it is as complex topic as solving captcha or fighting email spam.

You can check screenshots, you can check for new CSS/JS files included in page.

That's a start.

Also you should track DNS/NS changes.

Only screenshots are not enough. What if they redirect users elsewhere? Open new windows (well, browsers fight it themselves, but still).

To summarise this: - DNS changes - changes in included JS/CSS files - changes in new outgoing links on a given page.

This should be enough for starters