That's not a good rule of thumb since often the unique identifier you're interested in is a query arg, so if you strip of everything after the ?, you're left with nothing. like the URL above, all you'll have left is:

https://webcache.googleusercontent.com/search

Yes, there's no real general rule (see HN articles for instance) but "if I omit these parts of the URL, I get the same content" is usually a safe bet.