Ask the hotel clerk or concierge about the hotel's voltage. It is typically 220 volts in European countries and 110 volts in most others.
Check the outlet for plugging in electrical appliances and match the outlet to one of the adapters in a voltage transformer travel kit. The kits typically come with six types of adapters to fit most outlets a traveler is likely to encounter.
Make sure the adapter is rated for the specific voltage. For example, don't use an adapter rated for 110 volts in a 220-volt wall outlet even if it fits.
Plug the curling iron into the adapter and plug the adapter into the wall outlet.
Turn on the curling iron to preheat and use normally.