I only remember that he told Emma that he started changing (into wood) again when Emma was in Storybrooke. But how does he know that Emma was in Storybrooke? She could be anywhere. ;D Does anybody know?!

If I understand correctly, it was the moment the curse started weakening. The curse wouldn’t have started weakening unless Emma (the savior) was in Storybrooke. So, August knew that turning into wood meant the curse was weakening (why, I'm not sure?), and weakening curse meant that Emma was in Storybrooke.