It is very hard to know by just looking if a digital picture has been altered with a graphical editor or it is being presented on its original and raw format. Digital cameras embed all kind of metadata in the pictures they take, data about data found in the digital photographs inside EXIF (Exchangeable image file format).
JPEGSnoop is a tiny portable open source program that goes beyond extracting EXIF metadata, this free application can indicate the quality of the JPEG image compression used by the camera saving the file, JPEGSnoop uses the fact that each digital camera has different and specific compression quality levels when taking a picture and it compares that unique pattern against its large database of compression signatures reporting what digital camera or graphical editor was used to generate the image, being able to determine if a photograph is truly original or it has been edited with a graphics editor like Photoshop, software graphical editors unique compression signatures are included too in JPEGSnoop database.
JPEGsnoop reports countless of other details, EXIF / IPTC metadata, Huffman table decode, quantization table matrix (chrominance and luminance), chroma subsampling, JPEG saving settings, JPEG resolution settings, RGB histograms, etc. People who do not need that technical wealth of information about a digital picture can just skip it and look at the EXIF data instead, which normally includes basics like location, model of camera used and date and time.
JPEGSnoop can open .jpg, .thm (thumbnail), .avi, .dng (digital negatives), .pdf, .mov and extract embedded JPEG files from within them. If the photograph is corrupted and can no longer be viewed JPEGSnoop will try to recover the photograph thumbnail if there is one. This application can be of great use for home users wondering if that famous celebrity in the digital picture is a fake or real photo and it will also be of use for digital image forensic experts and amateur photographers alike.
There is other software capable of retrieving photographs metadata but JPEGSnoop stands out from the crowd by being the only one I know of able to tell you if the picture was edited with a graphical editor.