Error level analysis can help you work out if a photo has been digitally altered. Photos in a jpeg file format actually lose quality each time they are resaved. We can take advantage of this to try and work out if an image has been digitally manipulated.
Image Forensics : Error Level Analysis
Error level analysis allows you see to see the difference in quality level, represented by brightness. Things which are very bright have been edited most recently, whilst duller parts have been resaved multiple times. This allows you to see not just which parts of the image have been changed, but in what order the changes occurred.
> Go to Image Error Level Analyzer : Link
If the image has had no alterations to it, all parts of the photo will be at the same quality.