Question: How does a Stack Overflow moderator know about sock puppet accounts? 

How do they trap the user?

Answer: 
I am not going to answer this directly, for reasons that should be obvious.

However; this isn't a "trap" - it is our attempt at *enforcing reasonable behaviour*; using a sock-puppet for *any purpose* (most commonly, but not exclusively: upvoting yourself) is unacceptable.

If you want the vote of the masses; write good questions and answers.
Actions speak louder than IP address… Possibly even a “view” of a question is an action.