It's absurd that pre-release reviews are available to the general public. This should be easy to fix, if GR cared to do so.
Pre-release reviews is a problem on more than just books too. I wish there were more systems to verify that you've at least bought the book, movie, or the product in general before you can review it.
How can I prove that if I borrowed it from a friend or a library?
A number of copy-protection systems for mid-80's to mid-90's computer games asked that you opened the printed manual that came with the game and go to—e.g.—page 32 and type in the first word of the third paragraph. Doesn't seem that hard to automate doing that for ebooks, these days. After all, Goodreads is owned by Amazon so they have the corpus. Just use chapters instead of pages; that will work across all formats.
I'm not sure that would help with organized brigading campaigns like this. In the Internet age, one person with the book could supply answers to these questions for hundreds of people posting fake reviews with only a nominal investment of time.