画像のエンコーディングをJavaで検知するには？

ImageIO.getImageReadersを使う。

ImageInputStream imageInputStream = ImageIO.createImageInputStream(
    myInputStream);
Iterator<ImageReader> iter = ImageIO.getImageReaders(imageInputStream);
if (!iter.hasNext()) {
    // this always happens
}
ImageReader reader = (ImageReader) iter.next();
if (!reader.getFormatName().equals("jpeg")) {

}

http://stackoverflow.com/questions/4530348/detecting-image-encoding-in-java