It is quite obvious that the Hitchhikers guide to the galaxy is a Sci-fi book/film because it is an adventure around space and it has a load of alien life and a lot of crazy different planets that we don't know about. Another giveaway that it is a Sci-fi is actually some of the characters, Because a little bit through the book you meet Zaphod and he is a very strange character that is another clue that it is Sci-fi.