i usually can tell by the fin shape ,but as i said before i am no expert,and males seem to have more darker coloration and larger sizes(the female in the photo is 9 inches)
I'm merely going on general appearance and characteristics. For one, it really has a big round belly. And at that size, a male Synspillum usally has a nuchal hump and it's coloration has started to pop.
Buddha, the best way to sex it is to vent your fish. Flip it over and just before the anal fin you'll see what looks like a small circle and a V, or a small circle and bigger circle.
Male o <---anus
V <---genital
Female o <--anus
U <--genital
Like flowerpower said, take a close up pic of it's underside and we'll sex it for you.