Aye, true that it is. They need to be in the adult coloration before you can sex them. At 4 to 5" they are still in the juv. coloration, still gotta go through subadult. So you won't be able to sex them accurately till aboot 8 to 10" range...
. ONCE they do get to sexable age though, the main way to sex them will be to look for a white stripe in the dorsal and sometimes in the caudual fin. Females are the only ones that have this in Lugubris pikes. In Saxatilis pikes the females generally get a ocelli in the dorsal. Also, females will generally get a pink/red belly...