It isn't handled.  

If the user leaves a comment saying  

> Thanks so much!  It works!  

Then you *could* leave a comment along the lines of  

> If the answer solved your problem, please [accept it](https://meta.askubuntu.com/questions/1137/how-do-i-accept-an-answer) so others know your problem was solved.  Thanks!  

Sometimes that works, sometimes it doesn't.  But as a rule of thumb, it isn't a good idea to go bugging the user.