When are potatoes grown in containers ready to pick?

hmm from experience you really need to give container potatoes loads of water particularly now as the tatties will be swelling , if they are first earlies I would expect some sign of flowers appearing around this time . Do you know what type you are growing? and remember plenty water.

one way is to gently dig around the stem to see how the spuds are doing

another thought , have you had greenfly on the plants a can effect the growth and could stop flowers appearing