Here's my female jag that's the size of a 300 watt heater lol. How do I tell? I can't tell you. Only reason I know its a female is cause when I bought her about 5 years ago she was with her fry that she was raising. The owner said he just bred her and didn't have any room for her so I decided to take her for 25 bucks and he kept the fry.
Edit: I think you have a male, just by looking at the mouth I see a male imo. I'm probably wrong but that's what I think. Male jags are significantly smaller than females and females have a wider more pronounced mouths that almost look like underbites.