First and foremost, they'll have to be old enough to sex. Assuming that criteria is met, females will have large black marking(s) on the dorsal fin. This will be absent in males. Also, ripe females will exhibit a pinkish belly. Male will typically have longer extensions on their dorsal and anal fins. Males will also have numerous, small spots on the soft portion of the dorsal and anal fins.
Males also exhibit a strong herringbone pattern along their flanks.
Here's a male. Note the herringbone pattern and small, faint spots on the soft portion of the fins.
Dang, that is a NICE female sveni!! Mine didn't have that nice of splotches in her dorsal, but she was still pretty sexy in her own right! Basically like Scat said, in Saxatilis pikes, keep an eye out for the spots/splotches in the dorsal fin...almost a sure indicator that its a female. Its not fool proof though, as i have seen some where the male had some too...but generally thats the way it goes.
Don't get frustrated if you can't sex them at 4" either, as you probably won't be able to sex them until they are between 6 to 8"...if then. Sometimes you have to wait till they are almost full adults.
Last and formost, look for the red/pinkish belly...ripe females will definately have this, and unless the male is crossdressin, won't exhibit this...