If you had a short lead with a European plug on one end and a UK 13amp socket on the other you could use a standard Martindale polarity tester
In this country the polarity should be correct so once you have got it right it should stay right.
If you check polarity and it isnt right then you need to speak urgently with the site owner
Its worth checking in this country anyway because the Martindale will check earth continuity as well as reverse polarity
Its quite common on rallies to find a fault in this way as constant handling of the mains lead can cause terminals to lose their grip on the cable cores.
------------- John
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