We compare the list of followers with the list of those who has liked the post. If more than 30% of likes are fake (not from followers) it is a clear sign of cheating.
The process is not fast, but very useful for advertisers. We analyze each account follower, read his feed and find out in what places he most often put a geo-tag. Then we distribute the data obtained to the nearest major cities.
You can analyze accounts of up to 300 followers for free and without registration.