Suppose a non-Jew believes that there are multiple gods, but he does not worship them in any way. Is this a violation of the prohibition against idolatry for gentiles?
From http://www.moshiach.com/index.php?option=com_content&task=view&id=227&Itemid=64 which in turn references Nodah B'Yehudah, volume 2, Yoreh Deah, number 148.
So long as ascribing power to a deity other than the Creator remains conceptual, it is permissible to the Children of Noah according to many authorities. But worship of this independent being is clearly idolatry
Note: I have not checked the Nodah B'Yehudah, and nowhere does it say who wrote the book I copied from, so I hope, but can not be certain, that it's accurately representing what the Nodah B'Yehudah says.