Reversed polarity tester

Can anyone tell me how to use a tester plug to check for reversed polarity of campsite electricity hook-ups, as recommended for use abroad, please? The man in the shop said it should be plugged into any socket in the motorhome/caravan when first hooked up, and the order of lights will indicate whether it is OK or not. Seems straight forward - but, if there is reversed polarity, isn't there a risk that damage to the vehicle's circuitry will already have been done by plugging it in? In my ignorance I'd assumed it was possible to check the hook-up's polarity BEFORE connecting the vehicle!