How to identify load paths?
The simplistic explanation is that you design the aircraft shape, run computational fluid dynamics (CFD) simulations to get loads, then run finite element analysis (FEA) on the underlying structure to size the components.

CFD might look like:



FEA might look like:



Then when you have finished this sequential process you do it over again because after resizing the components the weight of the structure changes, so you have to change wing area to match the weight change, which makes the loads change, which then requires another component resizing.

You might wonder if it wouldn't be better if all these objectives were solved at the same time, and you'd be right. A newer design process is called multidisciplinary design analysis and optimization (MDAO) and is just becoming possible with modern, high power computing systems. 

MDAO might look like:



where all the red buttons are control points that the optimizer uses to refine shape. You have to give the optimizer rules on how to proceed, which might look like:



If you want to read about MDAO one good source is Univ Michigan at 

http://mdolab.engin.umich.edu/

Read a couple of their published papers and you'll get a pretty good sense of the complexity. MDO is the same thing as MDAO. 

If you are interested in the optimizer, OpenMDAO is open source software. It is not aviation specific, though it was developed in conjunction with NASA. It is at:

http://openmdao.org/