During the books, the great flaw of Sauron is depicted: his incapability to assume that anyone would prefer to destroy the ring rather than use it once it fell to his hands. To my opinion, there are some reasons to believe that he actually could foreseen it.
The first reason is that, as Elrond maintains, Sauron wasn't evil in the beginning. If so, it may be that even during his ascending in the end of the third age, a trace of good was still remained at him, slight though it be. I think that this trace was enough for him to feel some identifying with the desire of the people of the west to overthrow him, and to assume, due to their lesser numbers, that they would do anything to assure that.
Second reason: Gandalf himself told Frodo at Bag End that Sauron believed that the elves destroyed the ring. Only at his rise he learned that it is not true, so he probably believed in this for most of the third age, long enough for the false belief to be indeliable from his memory. If so, doesn't it likely that his first thought after his rising would be, "Ok, they didn't destroy the ring, but perhaps it's better for me to fortify Mount Doom and prevent them to do so, just in case"?
Third reason: The revealation of Aragron in the Palantír and the results of the Battle of the Pelennor Fields. In the chapter "The Land of Shadow" in book six, it is said that Sauron was pondering at the appearance of Aragorn and his sword Andúril, and "the Dark Power was deep in thought". Let's try to suggest what are these deep thoughts. I don't think that he was actually worried about Aragorn's number of soldiers, but it is possible that he saw his unexpected default as a first sign that he gets what he deserves due to his being evil. This thought could lead him to search for other ways to ensure his winning and survival, in addition to his military force, like a greek king consulting with the Oracle before his riding to war. Also, the realization that he has underestimated the numbers of the armies of Gondor and Rohan in the Battle of the Pelennor Fields, could make him realize that perhaps he underestimated their moral compass too, and that maybe they are interested in the destruction of the ring and not in the use of it.
I will be glad to read your comments.