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It depends whether the electrical item has variable voltage, and that in turn generally depends on how portable the item is.

So laptops, iPods, camera chargers etc. all tend to be variable voltage and only require a plug converter. Some larger items can also have variable voltage power supply, eg an Apple Time Capsule.

There's no short way other than checking each item.

Brookstone do a good, small combo converter adapter for smaller items if you only realize at the airport that you need it.