Minor spoilers ahead for Joker!
Joker is obviously not a typical comic book movie in that it’s lacking the typical hallmarks of the genre that fans have come to expect, debuting without a post-credits scene or many notable comic book references. However, both Joker director Todd Phillips and editor Jeff Groth confirmed to IGN the existence of a Batman Easter egg during the scene where Arthur Fleck visits Wayne Manor.
As Arthur (Joaquin Phoenix) approaches Wayne Manor, he spots young Bruce Wayne (Dante Pereira-Olson) playing on the front lawn in his gazebo-like jungle gym. (Seriously, what kid goes outside to play in a cashmere overcoat?! Only Bruce Wayne.)
Source: IGN.com Joker: The Batman Easter Egg You May Have Missed