You could detect similar texts but that's just an arms race against new corpora being added to their generator.
Maybe detect the same app getting a lot of similar ratings in a period of time? But then the farm could just randomize the input list of apps among the farmers and stretch out the time period to make it look like more natural traffic.
Thoughts?
I believe one would probably have to do the same thing to moderate app store rating.
All evidence points to Apple not really caring about abuse of this variety, so the obfuscation needed to pass any filters are pretty basic.
It would probably be better to cluster reviewers by apps they've reviewed, and to try to find patterns that way.
You could have some automated system which loads many different apps to be rated, so that the person in the picture isn't rating the same app over and over again in a short period of time.