I have been breeding severums for quite a few years now. I have found that Severums are one of the hardest fish to sex.
Yes sometimes they can be sexed by fins.
In most cases though it can be very unreliable. I've seen some of the best fish people I have ever met have some big problems with severums. Especially when they are juveniles.
Even one of my females was sexed wrong at one point. You should see the fins on her. Outstanding!
The only 100% way of sexing a severum is through venting.