Like title says, do we know how Nexus 9's are identified?
We know that Nexus 8's were identified by a serial number printed under their (left?) eye, and the VK test could also be used in the absence of the serial number.
The short film where Wallace introduces the 9 suggests they have a number, but if you look at it it's actually an NX8 number, and K in 2049 does not have a number (or willingly chooses not to look).
Wallace seems pretty confident in the 9's that I can imagine he would intentionally leave off the serial number as a symbol of his faith in his ability to create a truly obedient replicant.
The rule book has an emphasis on DNA/exegetic identification, as well as a DNA registry at Wallace Corp. The rule book also says the KIA has a bioscanner to read Nexus serial numbers... if the 9's don't have a printed serial number, can it scan their DNA to identify them? Or take their DNA samples to Wallace?
It's not explicitly stated anywhere that I can see, so maybe this is up to the game runners to decide. It's a bit inconvenient if the 9's all have numbers if you want to tell a story with a secret replicant or similar theme.