You can probably eliminate most of the simpler ways of watermarking an image via some simple thresholding operations with any image editing software. Make all the white regions completely white and all the black regions completely black. That will get rid of at least half the places where information could be hidden.
If there is a watermark there, and if you have the technical skills, you might be able to detect it by looking at the least significant bits (LSB) of the image.
Unfortunately there's no way to detect all possible watermarks without buying two or more copies using different accounts and comparing them.