By testing her ass.

"Stand up." You tell her. She confidently climbs to her feet. "Go stand in front of that table." You point towards Felicity's computer station. She walks over and stands there with her back to you. You approach her from behind, grab a fistful of her hair and bend her forwards. She instinctively puts her hands down on the keyboard to stop her face from smashing into the computer. She looks over her shoulder at you.
"Are you trying to make me angry or horny." She says changing her tone completly.
"You tell me." You say as you spread her ass cheeks and slide your cock into her tight asshole.
"Oh fuck!" She shouts, clearly not accustomed to the experience. You pull her hair again as you slide in deeper. She grits her teeth and moans loudly.
"Am I making you angry?" You ask.
"No." She says through gritted teeth.
You start to pound her tight asshole, but she doesn't give up. Every time you ask her if she's angry she says "No."
"How about now?" You ask as you thrust even harder.
"Actually I'm starting to enjoy it." She says, as if she's trying to annoy you now.