The first Flangetail species I met was Ictinogomphus decoratus (Common Flangetail) in Singapore and Cambodia. When visiting Sri Lanka, I saw a Flangetail again which looked pretty much the same but which was actually different: Ictinogomphus rapax (Rapacious Flangetail).
I struggled with the visual differences, there being little definitive help that I could find online. Though Sri Lanka has only I. rapax, their ranges do overlap in parts of southeast Asia and, I believe, India.
Subsequently I was given a copy of a paper from India that did describe the differences and this is my attempt to diagram them (as opposed to repeatedly following all the technical jargon).
Just because it is another Flangetail, I’ve now included I. australis, merely for interest, even though its range does not overlap with the others.
[There are other Ictinogomphus species (e.g. I. pertinax) not covered here.]