You have to be careful about trusting your instinct that a photo’s “somehow a bit off”. As human beings, once we get a hunch, we tend to subconsciously look for ways to justify it. I once saw a study in which they showed people a mix of genuine photos and Photoshopped ones, and asked them to decide which were which. Most subjects were highly convinced of their assessments, giving detailed explanations about features in the image. But in fact the people were no better than random at distinguishing real from fake.
Case in point: just the other day I saw on Facebook a video clip of a plane flying low over a built-up area. I live near Heathrow so this is a common sight for me and I thought little of this rather mundane recording. But evidently for a lot of people, seeing a commercial plane flying so low over residential houses was surprising to the point of ridiculous, and the comments were full of hundreds of armchair detectives calling “fake!”, each with his or her own explanation of why the shadows were wrong, or the trees weren’t moving enough, or the plane was flying too slow, or too fast, or the foreground was too in focus, or too out of focus, or any number of other often-contradictory justifications.
You could pick almost any photo on iNat and, if you stare at it for long enough, you’ll spot details that don’t seem quite to make sense (either biologically or photographically); and, if you’re not careful, eventually your doubts will cloud out reason. It’s just human. For now, some AI-generated images are comparatively easy to discern, but that’s going to be less and less true over time. Unfortunately relying on instinct to pick them out is not likely to be as effective a method as many might imagine.