How _did_ Hibiki Ryouga find out that Saotome Ranma was a student at
Furinkin High School and living at the Tendo Dojo? 

It's usually not too hard to find out what school someone attends. ^_^

As for knowing Ranma was at the Tendou Dojo, that would have been more an
achievement... had he known beforehand.  He didn't; note that he is
searching for Furinkan High School.  It's only after running into Ranma at
Furinkan that he learns where Ranma lives. ^_-

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