This question is dealt with by Fr. Anthony Nelson, in an article posted in PDF format: The Appearances at Fatima and Orthodox Christianity. There are some other articles of interest that are posted on the Mystagogy site: The Troubling Aspect of Catholic Marian Apparitions.
Speaking very broadly, any time a miracle is said to happen outside the Church, the Church obviously cannot say whether the miracle is true or false, unless God gives the Church special revelation on the matter, or when there is doctrinal error clearly associated with it. We should not dismiss out of hand the possibility that God may work a miracle for someone outside the Church, in any given case, but we should also recognize that even pious Orthodox Christians need to be on guard against spiritual self-deception... and those outside the Church can certainly fall into prelest, and do not have the benefit of the Orthodox spiritual Tradition which teaches us how to avoid such spiritual pitfalls. And so, while a non-Orthodox Christian may be completely convinced of some vision or miracle, we have to be aware that they may be sincerely deceived. Ultimately, it is usually not necessary for us to come to a conclusion one way or the other on such things, and so we can simply leave the question in God's hands.
