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.
I'm the admin for one of the rare instance of Stack behind a corporate firewall, which doesn't have votering detection - I've noticed this happening among a few co-located sprint teams. I created a d3.js Sankey Diagram to show the volume of people voting for other people and posted it on the site to let the community discuss it and it died down.
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.