How would you go about verifying someone rating a movie has seen it?
All you have to do is ensure that anyone posting anything on the internet uses their own real name and address, and has provided a copy of their passport/driving licence or other official ID as proof.
In order to be allowed to rate a movie, you would have to provide a copy of your cinema ticket.
In other words, anonymous online reviews are currently close to meaningless. Personally, I would rather read a couple of reviews by decent film critics, rather than take any notice of the number of 1* or 5* reviews.