I can sometimes tell males from females... but with this particular fish I cannot...
I can say I have females with fins that are as long and pointy as this one... so I don't think that's a valid way of sexing them...
The best way to sex them is to put it in with a female and watch the behavior. But since you said this isn't your fish that may not be an option...
But if you are considering purchasing it for breeding purposes, I would suggest against it. This fish has the "beak nose" which is a common and undesirable trait in Blue Dempseys which is believed to be something passed down to it's offspring.