User blog comment:SakuraKarakura/Ch 396: Explanations, Proofs, and Theories/@comment-1429792-20100319125812

"How did Aizen "see" the fight between Ichigo and Ulquiorra?"

There is nothing to suggest that he did see it. Aizen did not say that he saw it or knows what happened in it. He himself ordered Ulquiorra to stay behind & protect Las Noches. And he knew that Ichigo would end up fighting Ulquiorra again (he told Ichigo that Inoue was in the 5th Tower, as was Ulquiorra - therefore Ichigo would end up fighting Ulquiorra). But yet Ichigo is now in Karakura. Therefore, he must have beaten Ulquiorra and saved Orihime. He would have known that Ichigo was weaker than Ulquiorra, so if he beat him, then Ichigo must have grown in power. Note that on the last page, Aizen says "you seem to have somehow got stronger" (other translations are similar) - that means he is speculating - he does not have definitive knowledge of what happened.

Aizen is very smart - he just put the pieces together.