I blew up my wife's computer power supply here in Germany by forgetting to switch it to 220v. But then I blew up a German power strip that wasn't even on, and then two German irons that are made here. Turns out the apartment we were renting was running hot.

Now I'm afraid to blow up my meter in my new place or plug anything in...

How do I check the polarity of the 220v-to-110v transformers and my 110v appliances that aren't labeled, in order to ensure I don't blow up anything else? Also, does anyone recommend one of these over the other?

1. Using a 220v setting on a dual voltage appliance, like a desktop PC, with an adapter for the German 220vwall socket;
2. Using a 110v setting, plugged into a transformer.

It's a question of which thing you want to do the work of voltage reduction.