Any 240v appliance, low watt light bulb etc. Or volt meter of course. Most camper shops sell a plug with lights on the front to show the polarity. Many camp sites have the polarity the opposite. If too far from a shop, Amazon or the other one, cost about a tenner. If the campsite is wrong way, may have to swap positive and negative in your blue/white socket, to match the camp sites. Sounds a bit daft, but many small camp sites and foreign, are not really that bothered. There it is take it or leave it, attitude.