Sacraments are authentic if they are performed within the Body of Christ, which is the Orthodox Church. While we make no definitive claims about where the Holy Spirit works and where He does not, we believe that it is quite clear that there the Body of Christ is visible, it exists in the form of the Orthodox Church, and we believe that there are no sacraments outside the Body of Christ, as doing so would divide Christ and the Bible states that this just isn`t possible.
However, I should point out that this is a different stance from that of the Roman Catholic Church, which studies other churches (such as the Anglican Church), and produces documents stating that for such and such reason, their sacraments are invalid. We do not say that the heterodox sacraments are necessarily of no effect, just that as far as we are concerned, there are no sacraments outside the one church of Christ, the Orthodox Church.
At the same time though, obviously as humans we like to make judgments and have everything all nice and neat and understandable, so you do find people within the Orthodox Church with a more definitive stance: There are those who say that the Roman Catholic sacraments are valid, and there are those who say that the Roman Catholic sacraments are most certainly without grace (in my experience this is the most common view). I don`t think that I have ever heard anyone say that the Protestants have sacraments though. But I think most Orthodox could agree that as Orthodox Christians, we know we are receiving the Mysteries, and that what other people are doing doesn`t really concern us.