Its very hard to sex them without seeing them breed. The one that lays eggs first is a female...lol.
That line thing works sometimes, but if you happen to get a stunted discus or a fish with a higher body or something other then the "perfect discus" then i'd be shakey on that method.
Sometimes you can sex discus according to the strain if you see the whole batch of fish in the tank. Mostly with Red Strains it has something to do with the red being X-linked and the solid colored fish are females while fish with half color or something are males. But thats a whole different story.....
Some breeders, i am told, also look at the slant behind the pelvic fins. A more rounded space would be a female while a steep slant down would be a male. I'm sure its not 100% accurate but its another way. Using this method you're fish looks like a female to me.