I met Ictinogomphus decoratus (Common Flangetail) in Singapore and Cambodia. When we visited Sri lanka in March 2019 I saw a Flangetail again and it looked pretty much the same. However, this turned out to be Ictinogomphus rapax (Rapacious Flangetail). I struggled with the differences, there being little definitive help that I could find.
Subsequently I was given a copy of a paper from India that did describe the differences. Below is my attempt to diagram them (as opposed to following a lot of technical jargon). 😉
[Note: There are other Ictinogomphus species (e.g. I. pertinax, I. australis) not covered here.]