You might have seen many photos being shared on social networks like Twitter and Facebook, but the real fact is that not all of these photos are real. Most of the cases, these photos might be modified using any tools like Adobe Photoshop, but this cannot be found out just by viewing the image or checking its properties. FotoForensics is a free web service which uses Error Level Analysis (ELA) algorithm to determine if a picture or image is real or computer graphics, if it was modified, and even how it was modified.
You can simply upload the image and the service will check ELA and then show the analysis results. Once the picture has been analyzed, the result page shows both the original image and the ELA. You can even move the mouse cursor over to the original image which will turn into the ELA image for easier reference.
The resaved image was digitally modified: books were copied and a toy dinosaur was added. ELA clearly shows the modified areas as having higher ELA values.
You can see the results of the image modified above.
More details on how to interpret the results can be found here. The results from ELA are directly dependent on the image quality. You may want to know if something was added, but if the picture is a copy of a copy of a copy, then ELA may only detect the resaves. Try to find the best quality version of the picture.