I wait for the flowers then give it a couple of weeks. Then i choose a nice day to have a liitle dig with the hand trowel to see what size the spuds are. If egg size or bigger I dig.
We've been eating home grown for about 10 days now, ( East Anglia), so I think you are safe to have a look.
Should be ready interele. Make sure that they are watered well 24 hours before digging up, and just try one or two plant if in the ground. If in tubs, just try one. I emptied my spud bags 2 wks ago and have some lates on the go now.
If the flowers are still on them leave them alone, they are still growing. Once the flowers have gone and the plant has wilted and collapsed, they have stopped growing.
They can at that point be dug up ... or they will be quite happy in the ground, depending on what variety you have. Some store better than others, but the first potatoes of the year are not for storing as a rule, it tends to be main crop only.