How do you know that Shakespeare had an unhappy marriage?

Answer 1

You don't. Although Shakespeare spent much of his life living in London while his wife lived and worked in Stratford, this arrangement probably had more to do with economic necessity than marital turbulence. When Shakespeare retired, he retired to the home in Stratford which he had bought with his earnings as an actor and playwright to live out the rest of his life with his wife. She and their daughters paid for the monument to William in Stratford Church where Anne and William are buried side by side. None of this sounds like their marriage was unhappy. They just didn't get to see each other much.