'The first 8 bytes of any NCBI sra file is \'NCBI.sra\', and the file is binary.
For details about the format, see http://www.ncbi.nlm.nih.gov/books/n/helpsra/SRA_Overview_BK/#SRA_Overview_BK.4_SRA_Data_Structure'

def sniff(self, filename):

    try:
        header = open(filename, 'rb').read(8)
        if (header == 'NCBI.sra'):
            return True
        else:
            return False
    except:
        return False
