Not til the dawn had he heard it,
His face grew pale to hear,
How Bess the landlord's daughter,
The landlord's black haired daughter,
Had watched for her love in the moonlight—
And died in the darkness there
~Alfred Noyes, "The Highwayman"
Oliver knows Jack Force is a vampire.
Not because he's been studying the lore since he was six. Not because the boy is ungodly handsome and talented. Not even because he's a familiar.
When Schuyler is around Oliver, she tells him everything—all her fears and insecurities, her problems and opinions, her hopes and dreams, all the crazy things that cross her mind.
When Schuyler is around Jack, she is mute and shy, clinging to his side, eyes staring adoringly—emptily—up at him, nodding rapturously to everything he says.
Oliver knows Jack Force is a vampire, because he is sucking Schuyler's soul.