Try this in Python?

    import codecs
    
    fin = codecs.open('input.txt', 'rb', 'shift-jis')
    fout = codecs.open('output.txt', 'wb', 'utf-8')
    
    try:
        fout.write(fin.read())
    except ValueError:
        print "Not encoded using Shift-JIS!"

Not sure if that's what you mean - feel free to clarify, if not.