How do you you tell if a photo is photo shop?

It's hard to know for sure, but here some dead give aways:

1. Conflicting shadows (say one building has a shadow pointing right but a tree has a shadow pointing left)

2. Weird colors

3. Rigid (and obvious) lines (say your friend photo shopped huge muscles onto himself, but there is an obvious border between his skin and the muscles)

4. Distorted features (odd curves or blurry areas)

5. Conflicting reflections (if there is a reflective surface nearby, check it. If it reflects something different than the photo, then there is photo shop work)

Hope this helps!