I'm always a bit skeptical, but some things are more believable than others.
This quote from the bio on Kongming Archives of Xun Yu, as an example, seems completely ridiculous to me.
[Xun Yu said] "Yuan Shao’s troops are numerous, but their discipline is lax. Tian Feng is bold but too blunt to his superiors. Xu You is greedy and undisciplined. Shen Pei is devoted but stupid. Feng Ji is determined but selfish. The latter two men are left to administer to domestic affairs. If someone from Xu You’s family violates the law, they would definitely not spare him. And if they don’t spare him, Xu You would give to treason. As for Yan Liang and Wen Chou, they are simple men of prowess, and can be captured in the first battle.”
I have a lot of trouble believing that Xun Yu could predict all of this. How did he even know who several of these people were or what would bring about their failures/defections/whatever? I could understand, "Yuan Shao's army is strong, but lacks discipline and has problems," but not this kind of detailed analysis of all of Yuan Shao's officers' flaws. Even if Xun Yu had spies watching everything going on in Yuan Shao's lands, somehow, I'd find this kind of difficult to believe. There's also the fortune-tellers/old men that predict the fall of kingdoms accurately, predict that high ministers like He Yan, et. al are going to die, predict that Liu Bei will gain Hanzhong but lose its people and all kinds of other foresight that sounds good in a story but doesn't seem very realistic to me.