This is/was a problem on Stack Overflow (retired mod here). Work colleagues in the same office or company upvoting each other's questions or answers. You could often tell it was fairly innocent from the activity on their accounts (active, positive participation, asking and answering with reasonably good posts) and from the spread of votes (lots of upvotes given to/from unrelated users outweighing their votering count).

But sometimes the votering detection would ring bells and when contacted these users had genuinely not considered what they were doing was creating a votering, yet were willing to understand the problem at hand and back off from each other a bit.