Azuremain tells you everything you need to know about Scion to get in. Before Azuremain it was vague as shit and mostly in Kteng's. The new Planar Sanctum area can help you flesh out what you learn from Azuremain or come up with your own theories as well.
It's just the leaders do test you on PK theory if they can't think of anything else to ask you. The nature of the A/B/S system, how you defend the cabal in a hypothetical situation that's as close to 'you're screwed' as they can think of for your given class (usually involving one or two ragers or an out of range healer).
You can get into Scion as early as you want to but you may not always stay in. It's harder to get kicked out of Scion than Empire but it's not THAT much harder, especially if you're a mouthy fucktard or even go so far as to attack your own cabal mates. They've done it to me before, gotten spanked by me in PK but didn't die, and then were smacked down by an imm taking control of the archmage.