Skeptics.SE has several questions that are of the form "Is this specific image photoshopped?" - the last one today.
Sometimes, someone already did the legwork, and posted an analysis of the image on the web.
If not, would posting the image to Photography.SE asking whether there are signs of photoshopping in scope of the site (as opposed to generic "how to decide photoshop or not" like this one: How can I tell if a photo is real or faked?)