On Sat, 22 Aug 2009 12:33:58 +0100, Harry Bloomfield
wrote:

The only way to be certain is to disassemble and check. If the battery
is connected directly across the socket it might be possible to plug a
plug into it and check its polarity with a meter, but likely there will
be a diode in series to prevent wrong polarity, especially so if it is
unmarked.

You'd think so, wouldn't you? Especially on something expensive like
a Sony Walkman Pro? But there isn't. Big repair bill if you get it
wrong :-(