Currently, without having a rich experience in retouching there is probably very little chance you'd be able to tell. Even RAW files are, in a very basic sense editable.

Here are some hints:

• If you're looking at fashion photography, always be mindful of skin texture. Bad retouchers (trying to please clients) generally destroy skin texture but often forget about retouching the area between the upper eye-lashes and the eye-brows. Also, look for strands of hair which start in the head but suddenly disappear upon entering the face.

• The colour of skin is often very difficult to replicate if major work has been done, zoom out to a reasonable distance and try see if you can differentiate areas of unnatural colour.

• Inexperienced photographers tend to place a large emphasis on retouching a subject's eyes, pay careful attention to the pupils, irises etc. to see if contrast and brightness has been added. You can usually tell immediately if they look "too bright".

• More generally, it is often easy to spot if major objects in an image have been replaced by the Content-Aware Tool/Clone Brush etc. Look for areas of repeated pattern. Any pattern, be it large or small, repeated is a dead give away.

• Fake objects in an image generally have different light sources (Are you in a different Galaxy? Why are there two sources of sunlight?)

• Fake objects in an image also generally don't maintain the same grain as the rest of the image. They also tend to not have the same blurriness/sharpness as their surroundings.

I'll add some more if I think of any. Hope this helps.