Well, we know specifically from John 12:31 when Satan was cast out.
The context of Luke 10:18 is that the 70 are relaying to Jesus "We can cast out devils in your name." And Jesus makes this statement about "I beheld Satan falling from heaven." And this being the reason that they can cast out devils; because even at this point; He has dominion over Satan.
Now Jesus making reference to an event that hasn't happened yet, but He's seen it "in past tense"; is probably eluding to what He saw in "eternity" probably prior to being incarnated. Remember, the Godhead planned this entire thing out before creation ever commenced and in a very real way, outside of time that operates in the created world; there still is the "eternal domaine of God".
Jesus is the lamb "slain from the foundations of the world"; (Revelation 13:8) so everything that happens in time, is already happened outside of time. Kind of hard for us to wrap our brains around I know; because we are confined to time as it exists in the universe we live it. Time is a fixed "law" that only goes in one direction. "Eternity" though has it's own "rules".
Think of the created universe like a sphere that exists inside the eternal domaine of God. Inside the sphere, nothing created is omniscient, omnipotent, omnipresent, eternal or immortal; yet "everything" (i.e. God) outside of this sphere is.
So Jesus speaking of bearing witness to an event that hasn't happened yet in "spherical time"; very well could have been something witnessed of in eternity. Or of more probable "revelation" to Jesus "in the flesh"; probably was recollected to his earthly existence in a dream.
Psalm 139 talks about that. A lot of what Jesus came to understand about His life and purpose, came to Him in dreams. His dreams functioned psychologically to the same purpose ours do. They help our brains organize, process and make sense of our waking existence. This is why things we've been mulling over the answers to, sometimes the answer comes to us in our sleep and in the dream it's as clear as can be. (Now how muddy does that become upon waking? Sometimes it stays clear.)
So that is probably the more likely aspect of this "imperfect tense" verbiage.