For a long time you could tell if a picture was AI by looking closely. Older image generators left clear signs, and most guides still list them (see the checklist).
New models like Nano Banana, Midjourney and FLUX have fixed most of these. Hands look right, text is readable and the lighting holds together, so a careful look is no longer enough to answer "is this photo AI?". Screenshots and re-uploads also wipe the file details that used to give an image away.
The reliable way to check if an image is AI-generated is an AI photo detector that reads the pixels and the file at the same time. Phrasly does both and shows its evidence, so you see why an image was called AI, not just a score.