Originally Posted by: nowonder11... I just wonder if there is a way I could tell which position the switch should be in?

That switch connects the chassis to one side of the plug or the other, through a capacitor. This is intended to reduce the amount of hum. If the cap is healthy, it will work as intended, but if is shorted, and the polarity switch connects it to the 'hot' side of the plug, the chassis then becomes 'hot'. Which means the jack and the strings of your guitar are at line voltage. Touching anything that is grounded can then KILL you!

If you have an AC Voltmeter, you can check for voltage between the chassis and ground, before plugging in your guitar. Really, though, the only safe thing to do is get that two-prong plug replaced with a properly grounded 3-prong plug and cable. It sounds like you need to get the amp looked at anyway, so do yourself a favour. The life you save could be your own.