I've never had any problems with pollination, even in the greenhouse. I don't know if they are insect pollinated or wind, but if you want to be on the safe side you could do the paint brush or tap the branches to disperse pollen. It can be hard to tell when the flowers open, I usually only realise they have once I see very small grapes peeping out! Maybe someone who knows a bit more about grapes can give a better answer.
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