I'm no expert, but my neighbour says it is when the green shoots above ground start to keel over and die. I am sure there are lots of other ABers who have more expert answers for you, though!
It depends what variety they are. If they are earlies they can be lifted once they have flowered. Main crop are best left for a while after flowering so they can bulk up.
Early potatoes can be lifted approx 10 weeks after planting, scond earlies about 13 weeks and main crop from about 15 weeks, although those times are just a generalization as it depends on the growing conditions and the weather.
Hi
Thank you both for the info
I am growing spuds in a pair of old tyres in the greenhouse and they are now flowering so that was the reason for the question
again many thanks Dave G