Since Tumblr removed the ability to access raw images, it can be difficult to tell if a larger resolution version is a raw from their tumblr, artificially upscaled, or from some other source. Tumblr apparently keeps the original dimensions of the image even though they removed the access to the original.

Basically you take a tumblr post link like this one http://sunnysundown.tumblr.com/post/171811600611/the-mercyxtony-doodle

Remove /post/ from the url and replace it with /api/read/json?id=

Then remove everything after the post number.

So after all that you should get a url that looks like https://sunnysundown.tumblr.com/api/read/json?id=171811600611

Then use ctrl+F to search for "width" (with the quotation marks) to find the original dimensions. So in this case, the highest resolution image we can get is 1280 x 1527. However, the original resolution is 2462 x 2937.

Even though we can't get raws anymore, this can be helpful if someone saved one from before tumblr removed access and you can confirm that that is the original resolution.

at least artificial upscales can be recognized pretty easily since they always have very specific kind of quality damage on them.

A little update:

This doesn't work with blogs that are in safe mode. However, as long as their tumblr isn't deleted, you can view it in dashboard mode. You would then have to manually find the post and then either reblog it yourself or look through the reblogs for someone's account that isn't in safe mode.