How to check if InputStream is Gzipped?

It’s not foolproof but it’s probably the easiest and doesn’t rely on any external data. Like all decent formats, GZip too begins with a magic number which can be quickly checked without reading the entire stream.

public static InputStream decompressStream(InputStream input) {
     PushbackInputStream pb = new PushbackInputStream( input, 2 ); //we need a pushbackstream to look ahead
     byte [] signature = new byte[2];
     int len = pb.read( signature ); //read the signature
     pb.unread( signature, 0, len ); //push back the signature to the stream
     if( signature[ 0 ] == (byte) 0x1f && signature[ 1 ] == (byte) 0x8b ) //check if matches standard gzip magic number
       return new GZIPInputStream( pb );
     else 
       return pb;
}

(Source for the magic number: GZip file format specification)

Update: I’ve just dicovered that there is also a constant called GZIP_MAGIC in GZipInputStream which contains this value, so if you really want to, you can use the lower two bytes of it.