It seems like she’s putting on a show.
Unless this is your first time with this particular person, you should have a good idea how they act when they reach that point of no return. If her expressions and sighs seem different than usual or exaggerated, rather than breathless and random, she may be putting on a show. The same applies to the noises she makes. If you’re not trying anything new but her moans and grunts seem elevated, she could be faking it.