How to use this tool
To analyse an image simply drag and drop it onto the page (requires a modern browser like firefox or chrome). Then play with the quality slider to spot anomalies in the error level. The image I analyzed in the screenshot above is a picture of myself that I modified in GIMP. As you can see the error level on the fake part is quite significantly higher than on the rest of the image. There are no such anomalies on the original.
Having that said the algorithm is not exactly reliable, especially with images that have been rescaled and compressed often/intensely. So take it with a pinch of salt and feel free to have a look at the simple source code.