A new stopping rule for turbo codes, the input-output consistency (IOC) check is presented. It is based on an extended maximum a posteriori algorithm that also outputs (hard) extrinsic estimates on the coded bits. In parallel, the hard decisions of the info bits are re-encoded and if the two sequences coincide, iterative decoding is stopped. Remarkably, IOC beats other known stopping rules in terms of error rate and convergence speed, closely approaching ideal 'Genie'-aided decoding.