Yes. He knew that they existed and had a pretty shrewd idea of who possessed them (i.e. Gil-Galad and Galadriel). That's
why he made the One Ring; to control all 19 other Great Rings and thus, control all life on Middle-Earth.
Of the Rings of Power and the Third Age in the
Silmarillion has more information on this topic and the Great Rings pre-LotR

.