By all the rules I have seen over a couple decades:
N1 is the forward-most rotor. In the case of a modern front-fan engine this is the fan, which is directly coupled to the low-pressure turbine (or occasionally through a gearbox). In an aft-fan engine (CF700 or CJ805-23) it's the core rotor - compressor and HP turbine. In a helo it's the core again.
N2 is the second independent rotor the airflow sees. In a front-fan 2-spool engine, it's the core or HP rotor. In a helo or aft-fan it's the free turbine.
And for the Rollers, there's this N3 thingie. It's the core HP rotor, and the intermediate spool logically becomes N2.
So - simply follow the inlet air though the engine and count the rotors, and they will be in N1, N2, (N3) sequence.
There are other conventions too: Ng & Nf for Gas Generator (core) & Free turbine, or Nh & Nl for High-pressure & Low-pressure. These may be specified by a military user.