There are several things that make morphs obvious to my eye. I have extremely critical eyes though, probably because I've made morphs myself:

The fuzzy edges.

The colour dithering looking blurred on the breasts compared to the rest of the photo.

No shadows, or even worse, fake looking shadows. Simply adding black doesn't cut it. Look elsewhere in the photo to see how the surfaces are reacting to light and shadows. Usually skin goes from pale to reddish brown to near black. It's actually pretty rare when you get pure black appearing anywhere because of ambient and reflected light.

Cast shadow focus. Sometimes an object casts a shadow with a harder edge when it's closer to a surface. Then the farther away it gets, the more diffused the edge of the shadow becomes. I've seen some morphs that use a universal shadow fuzziness and it stands out and screams "I was painted in!"

Small breasts get round, but the shading stays the same. The rounder an object is the more dynamic the shading will become. When small breasts are enlarged and pasted back onto a photo, they look just as flat as before, they just take up more space in the picture. A simple rule is, the more a surface is facing away from the light source, the darker it will get. Of course ambient and reflected light plays into this. The same goes for hilights (the opposite of shadows).

No evidence of BE impact on others. No passed-out people with holes torn in their jeans from orgasticular explosions. This is optional of course.

Vitamin "C"