Several quick rules of thumb:

1. Almost everything has a sticker or imprint that lists the input voltage. Look at that.

2. For the more electronics savvy, you can usually just go on whether you know the device already requires voltage conversion. Computers, phones, and other things with charging batteries almost always require a switching power supply to convert the voltage to something much lower than grid power. It's usually not too hard for the manufacture to make these work with multiple voltages.

Several quick rules of thumb:

1. Almost everything has a sticker or imprint that lists the input voltage. Look at that.

2. For the more electronics savvy: if you're just trying to do a quick inventory in your head you can usually just go on whether the device already requires voltage conversion. Computers, phones, and other things with charging batteries almost always require a switching power supply to convert the voltage to something much lower than grid power. It's usually not too hard for the manufacture to make these work with multiple voltages.