I'm never sure myself - I think you have to wait for the flowers or plant to die off then thepotatoes are ready for digging up.
I don't usually have the patience to wait for this so I usually have a little rummage about in the soil and see how they are getting on. Just try not to disturb them too much in case they aren't ready.
there was something about this in the Observer yesterday - I think esselle is right and its the flowers dying off.
Here you go:
Early spuds should be ready for cropping this month. Wait until the first flowers have opened before harvesting, and water well if the weather is dry, to encourage good formation of tubers. Keep vigil for blight if the weather is wet. Leaves will rapidly collapse if it hits. Cut off all foliage and burn. Do not compost. The tatties might well be fine under the soil if they are already formed and you strike fast.
what variety are they and when did you plant them?
you can earth them up to get more potatoes too... (basically put earth round the stems leaving the top 4 inches of foliage clear).
i normally wait until they've died down completely but if you are desperate to eat them, cut a hole in the bottom of the bag and fish around for them from the bottom, that way the ones at the top (which may be smaller) will carry on growing.