How do you know Albus Dumbledore is gay?

The author of the Harry Potter series, J.K. Rowling, announced that information. Albus Dumbledore was in love with a friend from his teenager years, Gellert Grindelwald. These feelings were not returned.

In 1945, Dumbledore duelled Grindelwald and defeated him after he became a dark lord. Grindelwald was then the master of the Elder wand and after his defeat, Dumbledore became the master.