Check the edges of the picture when in doubt, and something seems fishy. You can zoom in for that if the mistake is not that obvious. Honestly, I find photoshop scary when it concerns sensitive topics and fake news.
Reversed text is another dead give away. It seems obvious, but if the text is mirrored or flipped in one part of an image, but then it's correct in another part of the image, then you have a good case that the image was doctored.