Spoilers: Through episode 2.22, "Over There" part 2.
Disclaimer: I don't own Fringe or its characters.
Pretending his whole focus is on their flirtatious banter, Peter studies Olivia. She's relaxed as she sorts through paperwork, almost slouching, and the slight grin makes her seem younger. She's certainly more carefree. More happy.
He's ashamed it took him three days to figure out the truth.
They were chaotic days, full of mixed feelings at being back in this Boston, and he hasn't seen much of her. They've only had a few hours together, here or there, as he settled back into this reality and she settled back into a job that had apparently been all but put on hold by her search for him. Those are excuses, though; he should have known immediately. The walk wasn't quite right, the tone wasn't quite right. Almost, but not quite. He'd been arrogant enough to think that it had something to do with him, that at last he was seeing what Olivia looked like when she was truly happy and unburdened.
It took three days to come to his senses, to realize what he should have known instantly: this isn't Olivia. Not his Olivia, anyway. His Olivia was the one who poured out her heart to him then kissed him like she was afraid he would turn her away. The one who came back, the one who boldly flirts and who turned down his offer of a beer with a twist of her mouth showing her distaste, is not her.
Three days, when he always prided himself on knowing her so damned well.
In the end it took that one difference for him to stop being an idiot and figure out the truth, when it should have been a thousand differences that clued him in. She crossed realities to find him, crossed realities to save him, and he hadn't even recognized when he'd lost her. He has no clue if his Olivia is dead, or alive and suffering, and fury burns through him as he imagines all the possibilities.
She has to be alive. There is no other option.
He doesn't know if it was Bell who engineered the switch, or if this imposter fooled both of them. But whoever is behind it, this Olivia is here on orders. And given Peter's reaction to hearing the name Olivia Dunham in his father's office, his father knows he cares for the Olivia he left on the other side.
He remembers his father. Not the sweetness and light version who brought him back, but the one from dim memories of his childhood. If she was captured alive, she'll stay that way. His father will pull every bit of information about this side out of her, by any means necessary. And then he'll use her against him.
His father now has most of the cards, but Peter has one ace in the hole. Peter knows that the Olivia who travelled back with him is an imposter, and that his Olivia is trapped on the other side. Now he's going to use that knowledge to play the most important con game of his life. If he can convince this Olivia that he knows nothing of the switch, he might be able to gain her trust and use her to get his Olivia back.
Because he's going to get her back. Or there's going to be hell to pay.