Its normally either when they've flowered or for varieties that don't, when the tops begin to die back. Its rather late for a crop this year, though you might get something in view of the very mild autumn.As its now November I would dig gently round one of them and see what's underneath. If there are any there I'd salvage what I could.

I'd be very surprised if you got a crop off potatoes planted a few months ago but, as Stera suggests, have a feel round and see if you can find any reasonable size tubers. Don't expect them to grow to the size tatties planted in the spring do.