How do they know the 2016 Kaikoura (NZ) Earthquake was 'complex', involved six different faults?
You need to see the InSAR data. E.g., see this image which is based on radar data acquired by ESA's Sentinel-1A satellite. For straight faults the pattern is not this complicated. 

Using such observations we can estimate which parts of the fault slipped. E.g., take a look at these images. This is done by solving an "inverse problem", i.e., you have some observations and you're trying to estimate what parameters in a (physics-based) model can reproduce the observations. In essence, it is an optimization problem like curve fitting.

Note: Besides InSAR data, we can also use data from seismometers but the former makes it rather obvious.