Hi Michael,
The best way to tell the difference is going to be experience and after a while it will be fairly easy to tell the difference. What you'll notice is that South Sea pearls have a softer luster the Freshwater pearls, much more like satin. You will also notice differences in the color of the 2 pearls, with silvery body colors being common with SS pearls.
I've attached an image of South Sea pearls and an image of Freshwater pearls as a reference. The top one is a pic of the most common Freshwater pearl colors and the bottom are common colors found in South Sea pearls.
I would suggest you visit some jewelry stores to get a chance to see more strands close up and also visit online stores as most are filled with pictures of pearls.