That was long after Spica made the decision to roam top instead of drake.
I watched the play back. Spica diverts from reading toward drake once he sees Thresh/Olaf kill that control ward. So his tell is only that a ward was killed and uses that to say, "Yeah, this is probably a crossmap." It's not mind-reading, but it's still a very smart play to intuit that dead ward = top dive because the rest of their run toward the turret is through fog.