when do you know that your patotoes are ready to dig?

"Tickle" them. Gently burrow into the soil until you find a spud or two and check their size.
If they're "Main crop" you can almost wit ubtil the tops are on the wain to let them grow to max size.
HTH
Sir Sid O'Fillum.

They say (books etc) that the earlies are ready when they come into flower but that's not always reliable. As Brainake says, if possible, have a little root around. Sometimes you can even (if you're careful) pick one or two and leave the plant in place so it will produce some more later on.(ones that are only tiny now). It can be difficult to do this though. One tip I've seen recently is to plant potatoes in stacks of old tyres(adding a tyre when earthing up) and you can carefully remove one or two tyres to see how they are getting on That's a good idea if you're only growing a few but would take a lot of space and old tyres if you wanted to plant a lot of tubers. Good luck. I hope your first crops are successful. I bet you can't wait!
Paul.