A blink comparator was used.

> The photographic plates Tombaugh was comparing with this machine were 36 x 43 centimeters (14 x 17 inches), and were long exposures taken with a telescopic camera that sported a powerful 33-centimeter (13-inch) diameter lens.

https://airandspace.si.edu/stories/editorial/finding-pluto-blink-comparator

They would switch back and forth between multiple photos of the same spot. Makes it way easier to see differences. Although I do believe he went through hundreds, if not thousands of images looking for a tiny dot moving.

There's a website where you can use similar images to try and identify new objects. I think it's called Planet Nine or something. But the images in this post are much better resolution than the ones on that website. And I still can't tell how they could identify Pluto from these images.

Where's Waldo but instead its Where's Planet