The phenomenon of neural networks synchronization by mutual learning can be used to construct key exchange protocol on an open channel. For security of this protocol it is important to minimize knowledge about synchronizing networks available to the potential attacker. The method presented herein permits evaluating the level of synchronization before it terminates. Subsequently, this research enables to assess the synchronizations, which are likely to be considered as long-time synchronizations. Once that occurs, it is preferable to launch another synchronization with the new selected weights as there is a high probability (as previously shown) that a new synchronization belongs to the short one.