The aircraft "knows" nothing. It relies on appropriate air data and "intelligent" pilot inputs to perform within its design boundaries.
AF447 did all it was designed to do, and when the "intelligent being" making the command inputs wanted NU, the aircraft complied.
What bothers me is the "intelligent" part of this equation was missing.
I doubt if any of the pilots on this thread would want the aircraft designers to put the "intelligent" part into the automation so that the aircraft "knows" best.
The degradation of control laws is indicated on ECAM as well as on PFD.
On ECAM
in ALTN : CAM EW/DFLT CTL ALTN LAW (PROT LOST)
MAX SPEED 305/.82
in DIRECT : CAM EW/DFLT CTL DIRECT LAW (PROT LOST)
MAX SPEED 305/.80
USE MAN PITCH TRIM
On PFD
The flight control status awareness of the crew is enhanced on the PFD.
Indeed the availability of PROTECTIONS is outlined by specific symbols = (green), and by the specific formatting of the low speed information on speed scale, in normal law.
When protections are lost, amber crosses X are displayed instead of the green protection symbols =.
When automatic pitch trim is no longer available, this is indicated as USE MAN PITCH TRIM amber message below the FMA.
Auto trim is always available except when indicated to the contrary as detailed above.
As I have posited previously, the PF effectively spent 4 minutes battling with a compromised yaw damper that contributed to the roll and at no time was the SS left in the longitudinal neutral position. Without regurgitating the stuff long since posted (many times), if you simply don't know, the result will be equally simple.