There is a lot of information to draw on when looking for fake recommendations - it isn't all down to IP, due to the reason you give - that people can be seen tweeting and reviewing almost as they eat - obviously using the restaurant IP. Ten years at this next year and I can 'feel' a dodgy post even before I have read it sometimes and it's often down to the English, or tiny pieces of punctuation, or a far too "over the top" review where absolutely everything was perfect. I've never been anywhere that was perfect on every last detail..!
There's something blindingly obvious about a fake review. I've asked other members of the team their opinion on a first post review that is very positive about a restaurant but after all is said and done seems absolutely genuine.
The one thing I wish these people could grasp is the damage they do to their reputations. It's like they haven't managed to get a positive review from a genuine customer so they have to pretend to be their own cutomer. It is better to have no review than fake one...