How did Yoda know that Anakin killed the younglings before watching the security hologram?

Question

In Episode III, before Obi-Wan started watching the security holograms, Yoda already warned him:

YODA: If into the security recordings you go, only pain will you find.

Obviously he was talking about Anakin... But how did Yoda know in advance that Anakin killed those younglings? Obi-Wan was full surprised to see that!

Answer 1

An earlier scene in Episode III shows Yoda reacting quite negatively as Anakin first takes a dip into the Dark Side. It's likely he already suspected from this that Anakin was turning evil. I believe it was at the exact time Anakin was killing the younglings.

He may not have known exactly what happened (like how in Episode IV, Obi-Wan knew something terrible had happened, but not yet that Alderran had been destroyed), but given the events that had transpired, coupled with this feeling, he may have been able to put the two together.

Answer 2

In the novelization it is clear that Yoda does know who did it:

Obi-Wan stepped over white-armored bodies to kneel beside blaster-burned corpses of children. Tears flowed freely down tracks that hadn't had a chance to dry since he'd first entered the Temple.

"Not even the younglings survived. It looks like they made a stand here."

Yoda's face creased with ancient sadness. "Or trying to flee they were, with some turning back to slow the pursuit."

Obi-Wan turned to another body, an older one, a Jedi fully mature and beyond. Grief punched a gasp from his chest. "Master Yoda—it's the Troll..."

Yoda looked over and nodded bleakly. "Abandon his young students, Cin Drallig would not."

Obi-Wan sank to his knees beside the fallen Jedi. "He was my lightsaber instructor..."

"And his, was I," Yoda said. "Cripple us, grief will, if let it we do."

"I know. But... it's one thing to know a friend is dead, Master Yoda. It's another to find his body..."

"Yes." Yoda moved closer. With his gimer stick, he pointed at a bloodless gash in Drallig's shoulder that had cloven deep into his chest. "Yes, it is. See this, do you? This wound, no blaster could make."

An icy void opened in Obi-Wan's heart. It swallowed his pain and his grief, leaving behind a precariously empty calm.

He whispered, "A lightsaber?"

"Business with the recall beacon, have we still." Yoda pointed with his stick at figures winding toward them among the trees and pools. "Returning, the clones are."

Obi-Wan rose. "I will learn who did this."

"Learn?" Yoda shook his head sadly. "Know already, you do," he said, and hobbled off into the gloom.

What is not very clear (at least not in absolute and digested terms at that very moment) is exactly how he knew. But, throughout the novelization and the movies Yoda's distrust and general "bad feeling about this" towards Anakin is very apparent. He knows what Anakin is capable of through his inability to control his anger ever since he was a child. He also knows that Anakin has mastered the lightsaber to an extent where he would easily defeat Cin Drallig (Yoda's own student) who was protecting the children. So it is not entirely inconcievable that knowing a lightsaber did the killing, Yoda would put two and two together and come up with Anakin as the culprit.

Answer 3

A quote from Episode III came to my mind regarding to this matter.

When Anakin wakes up in his loft, after another nightmare, he asks Padme:

Anakin: Obi-Wan was here, wasn't he?

It may lead us to assume that a Force sensitive must be able to identify one's unique presence, even after he left. That would be enough to conclude that Yoda might have felt Anakin's strong dark presence at the temple.

Answer 4

An interpretation which does not require the Force to explain Yoda's knowledge: Yoda may have watched the recording while Obi-Wan was disabling the beacon.

If you watch the scene carefully, you'll notice that Yoda was standing in the room where the recording was while Obi-Wan was finishing with the beacon. Yoda would have had time to view the recording while Obi-Wan was busy with and distracted by the beacon.

The other answers argue that Yoda put two and two together and suspected or concluded that Anakin was the culprit as a result of Anakin's flirtation with the dark side. This is certainly a possibility, but would not necessarily offer the definitive proof of Anakin's guilt that the security recording did. Moreover, Obi-Wan would be just as aware as Yoda -- if not more -- of Anakin's dark side tendencies so it is not clear why only Yoda would be able use this knowledge to conclude that it was Anakin.

In case you don't have the film readily available, the relevant clip is on Youtube.

Note: I originally posted this as a comment to the question, but I got some upvotes and other comments of agreement on it so I've promoted it to an answer.