Which sentence states a fact?
A: Harry Houdini, born Erich Weisz, used "Houdini" as his stage name to pay homage to famous French magician Robert Houdin.
B: Harry Houdini was a mediocre magician, but many people considered him to be a skilled escape artist.
The first sentence states a fact. It can be verified by looking up how Harry Houdini chose his stage name in a biography or Houdini's own writing.
Harry Houdini, born Erich Weisz, used "Houdini" as his stage name to pay homage to famous French magician Robert Houdin.
The second sentence states an opinion. Mediocre indicates a personal judgment.
Harry Houdini was a mediocre magician, but many people considered him to be a skilled escape artist.