Not much info out there. I do not think there is any sort of external way to tell, but you could probably tell the difference during a spawn. The sperm would look like milk and the eggs (I think) would look like goo with tiny spheres in it.
If you have had a healthy magnifica for a long time and it has never spawned, it is possible that you have a female and magnificas don't release their eggs into the water, but rather develop the babies internally.