Probably mid 1800s when steel started to be mass produced in what is noted as the 'Second Industrial Revolution' because it set off a period where we could produce more bigger things faster, more numerously and efficiently .... it coincides with creating and producing more and larger coal fired factories ... and we've never stopped.

The other starting point would be the mass production of plastics between 1910 to 1940

Another marker would be radiation specifically July 16, 1945 with the detonation of the first nuclear weapon on earth. Apparently, this is an actual marker used by art historians to date original works of art. A forgery can easily be detected by the appearance of minute amounts of increased radiation in every material after 1945. Original works of art can be identified by the lack of radiation embedded in the art work because they were produced during an era when there was not as much radiation in the atmosphere.

But generally speaking ... in a thousand years or ten thousand years ... I think they'll look back at the 19th century as basically the starting point of the Anthropocene