I think he knew it was probable. After all, he created all living beings, and that meant designing their minds and how those minds work. Why do I think it was probable? Because in the Bible, there are a few times when perfect beings break his rules. Obviously the serpent did, which means that the angel who used it to deceive Eve also turned his back on god. Adam and Eve, two perfect humans, disobeyed the single rule they were given. Later on, other angels left their posts in heaven to have sex with human women. Revelation implies that as many as a third of the angels fell along with Satan.
When that many perfect beings cannot follow god's rules, it's pretty clear that there is an issue with the design, or that the design was intended to have this effect. So the odds were in favor of Adam and Eve breaking god's rules.