Manipulation of images are detected by looking for the following clues:
1: Shadows - Compare different objects and the direction of the light source - round objects are good.
2: Reflections - See if they all match up.
3: Eyes - Zoom in and compare against other eyes. (Dots/colors give light direction)
4: Clones - Can you spot a duplicate patch?
5: EXIF - Look for hidden clues in EXIF file data. (GPS Position data, Time, RBG Color Profile Changes, Compare Image Compression Signature)
6: ELA - Photo Analysis. Error level analysis (ELA) works by re-saving the image at 95% compression. Then calculating the difference. Big changes indicate its a manipulated. Touched up areas are easily seen. Blue/Red areas are generally Photoshop at work. Read the tutorial below for details.
FotoForensics - Photo ELA Error Level Analysis Image Tool
Original Developer: http://www.errorlevelanalysis.com/
Jeffrey's Exif Viewer - Online EXIF data and GPS viewer with Map!
JPEGsnoop - Fake image detection via image signature analysis
IEXIF 2 - PC based EXIF data viewer (includes GPS)
Advanced Reading: http://up.stevetjoa.com/stamm2010icip.pdf
To get a better idea on how easy it is to manipulate an image try this tutorial with audio.
It will be the best 5 minutes you ever spent watching YouTube.
Removing / Creating / Filling / Faking / Objects / Images in Photoshop CS5 using the Content-Aware Fill tool: