How to identify contents of a byte[] is a JPEG?

Question

I have a small byte array (under 25K) that I receive and decode as part of a larger message envelope. Sometimes this is an image, furthermore it is a JPG. I have no context

Answer 1

Some Extra info about other file format with jpeg: initial of file contains these bytes

BMP : 42 4D
JPG : FF D8 FF EO ( Starting 2 Byte will always be same)
PNG : 89 50 4E 47
GIF : 47 49 46 38

some code:

private static Boolean isJPEG(File filename) throws Exception {
    DataInputStream ins = new DataInputStream(new BufferedInputStream(new FileInputStream(filename)));
    try {
        if (ins.readInt() == 0xffd8ffe0) {
            return true;
        } else {
            return false;

        }
    } finally {
        ins.close();
    }
}