Recently, I published a paper regarding this topic:
A simple method of identifying π orbitals for non-planar systems and a protocol of studying π electronic structure
Tian Lu, Qinxue Chen, Theoretical Chemistry Accounts,  139, 25 (2020) DOI: 10.1007/s00214-019-2541-z
https://link.springer.com/article/10.10 … 019-2541-z

in which the algorithm of identifying pi electrons for any kind of system and studying their characteristics is detailedly described, and many interesting research instances are given, I highly recommend Multiwfn users to read this paper.

In addition, if pi electron analysis module (subfunction 22 of main function 100) of Multiwfn is employed in your work, please not only cite Multiwfn original paper but also cite the aforementioned paper, thanks!