'Returns the offset of the flac block start
(skipping id3 tags if found). The passed fileobj will be advanced to
that offset as well.'

def __check_header(self, fileobj, name):

    size = 4
    header = fileobj.read(4)
    if (header != 'fLaC'):
        size = None
        if (header[:3] == 'ID3'):
            size = (14 + BitPaddedInt(fileobj.read(6)[2:]))
            fileobj.seek((size - 4))
            if (fileobj.read(4) != 'fLaC'):
                size = None
    if (size is None):
        raise FLACNoHeaderError(('%r    is    not    a    valid    FLAC    file' % name))
    return size
