You can't tell.

"If you want, we can switch places. I wouldn't mind laying back while you did all the work," you tell Kako.
"Oh I would, but my butt needs practice first. Besides, you are still making progress albeit slowly. Stand up a sec, let me reposition myself." You rise up slowly, until you feel Kako's heavy breasts succumb to gravity and slide from you. You remain squatting, as you are sure she will enjoy the view. Within seconds, you feel her lift her cock upright until the long leaky length rests between your legs, right behind your balls. Certain she is finished with her repositioning, you let the cock slide back between your cheeks as she once again pulls her tits into place. You can feel yourself catch on the base of her cock as you pull upwards to encompass her tits. She helps you by pulling her tits by the nipples up into you. This time she doesn't let her arms go inside, instead releasing her tits. When she does, her breasts push outwards. You gasp as your butt pulls hard on the base of her shaft and the two mammoth mammeries in an attempt to pull them closer together. Her hands rest on your hips this time, since they are free. You can feel her pull gently on you, guiding you downwards. After a bit, her hands travel up your sides. From the tops of your flared hips, they quickly bump into the bases of your recently active breasts. She tickles them a bit and then futilely tries to cup them. She feels the sides of your boobs on her way up. Then she rests her hands on your shoulders and encourages you from there. You start to feel yourself widening as she pulls you downwards and you envelop her swollen middle. You cringe as you think you may have gone a bit overboard there. Despite how much you think it should hurt, you find it is not as bad as what you have already done taking her from cock to tits. You are already used to her pulling your cheeks wide apart to stuff you full of herself. Her effort and skill pays off as your cheeks not only touch the floor, but as you sit fully on them. You pull your tits apart and look down between them and see the hump of your middle looking fine with the large cock pressed up against the outside. You rest your hands on Kako's balls and rock back and forth.
"Is that enough stimulation?" you ask her.
"Not as much as I'd prefer, but the kink will get me off soon." Kako runs her hands from your shoulders down to the sides of your boobs again and runs them along the ample curves. "Can you push them together with your arms?" You pull your arms inwards, which causes your cleavage to expand upwards, and she runs her hands upwards to feel your cleavage bump into your chin as you look down. You watch her as she pushes her hands between your breasts. "Ooh, that's some tight cleavage you got there."
"Well, they already press together firmly due to their size when I'm not pushing them together," you inform her.
"Cool, I'll have to titfuck you more often then," she giggles.
Kako seems content with the status quo.