Put the picture through one of those AI rescaling things and double the size, then take a regular editor and halve it back.
Then put the original and the rescaled pictures on top of each other on separate layers and subtract them ("difference" in Photoshop). The result should be almost black with some strange frilly edge effects and compression artifacts.
Then put an adjustment layer on top, and crank the levels (or curves) up to make the dark values brighter. If the watermark shows up, it is present in the original but smudged out in the rescaled image. If you see areas that look like repeating text but you can't make it out well, then the watermark is still there, just slightly damaged.
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