It is so interesting to me that Sansa would root for the Hound and not Ser Jaime (the knight incarnate) here. And you can tell that Sansa is rooting for the Hound, too (not just making an idle comment) because she gasps when the Hound is nearly unhorsed at the first pass. Ned describes Sansa as watching this particular joust “all moist-eyed and eager” and “so engrossed she scarcely seemed to notice his arrival.”
Okay this got super long so the rest is under the read more!
(Spoilers for the first four books.)
So here is my attempt to explain what’s going on here: Sansa likes to relate to people through set roles taken from stories. An example of this: a few chapters later she cries when her father doesn’t send “the hero” Ser Loras after “the monster” Ser Gregor to bring him to justice.
When the Knight of Flowers had spoken up, she’d been sure she was about to see one of Old Nan’s stories come to life. Ser Gregor was the monster and Ser Loras the true hero who would slay him.
This is why she doesn’t get along with Arya - Arya doesn’t conform to the highborn lady role in Sansa’s story. This is why she looks down on King Robert - he doesn’t quite match her mental image of a king. This is why she looks up to Queen Cersei - she is exactly Sansa’s mental image of a queen. This is why she is so lost when it comes to Joffrey. This is how she is able to assign herself a new identity based on weird generalizations she’s heard about bastards from stories - she relates to herself through a different story role (“the bastard girl raised in a sept”) the same way she relates to others through story roles, and it’s relatively easy for her.
So what does this have to do with “I knew the Hound would win?”
Sansa heard the Hound’s story about his face the night before, and she sympathized with the story enough to try to comfort him. Obviously, the story made an impact on her and was added to her little arsenal of stories, because she sees Gregor as a hideous monster from then on. Sandor’s role in the story that she is making up is a little more muddied, and changes depending on his proximity.
When Sandor isn’t around, she romanticizes him, thinking of him (especially after he saved Loras from his brother) as someone who would never let her come to harm. But whenever Sandor opens his big stupid mouth and says things like weak people are mutton for wolves, she has no choice but to try to reconcile her romanticized image of him with the reality of him being a scary rude horrible brute, and it hurts her feelings in the same way that Arya refusing to be a lady hurts her feelings.
It’s the reason why she doesn’t approach Sandor after he saves her from the mob - his actions are the single thing that matches up with the role she’s given him, and it’s easier for her to think about “the Hound” as someone who selflessly saves her life when his terrifying self is not around to belittle everything. Even the way he saved her life was too violent to quite match up with her story of “maiden being saved from danger,” what with her scandalized description of his transformed terrible burned laughing face while chopping a guy’s hand off.
You can see her romanticism clearly in action when she gamely tries to thank him the way she thinks people should be thanked after saving young maidens, but Sandor makes it incredibly difficult. There’s a very clear discrepancy between how the conversation actually goes and the way Sansa thinks it should go.
“I… I should have come to you after,” she said haltingly. “To thank you, for… for saving me… you were so brave.”
“Brave?” His laugh was half a snarl. “A dog doesn’t need courage to chase off rats. They had me thirty to one, and not a man of them dared face me.”
She hated the way he talked, always so harsh and angry. “Does it give you joy to scare people?”
"It gives me joy to kill people."
and a bit later
Sansa hugged herself, suddenly cold. “Why are you always so hateful? I was thanking you…”
"Just as if I was one of those true knights you love so well, yes."
Her first instinct is to romanticize his actions, but it’s hard to do so with him being such a dick. This explains why her romanticism of him gets more pronounced the longer they are separated - he’s not around to correct whatever mental image she wants to have of him.
So yes. She knew the Hound would win. Because the Hound had just been given an interesting place in her covey of story characters the night before. I suspect she wanted to see the Hound joust against his evil brother the Mountain like in a story. I know I did, after hearing the story he told her.
Also, another thing about that moment - I love how Littlefinger overhears her comment and says, “If you know who’s going to win the second match, speak up now before Lord Renly plucks me clean.” This is the first time Sansa outguesses Littlefinger or knows something Littlefinger doesn’t, and I find it super cool (and maybe even a bit of foreshadowing) that it involves Littlefinger underestimating Sandor and Sansa estimating Sandor correctly.