For now, nothing, unfortunately – which is different than what would happen in the usual Keras model: in the usual Keras model one would get a history of evaluations on the return.
For now we briefly documented the difference (see fit method returns) 3, but we are already working on fixing this – it should be coming in the next few days (now I notice we should have documented on validation_data argument, as well as on the various models).
The simple work around is for now call model.evaluate() on your validation data. Notice that DF only train on one epoch, so one will only get one evaluation on the validation dataset anyway.