Step,
I think if you are buying based on photos, it would help you to look a lot of photos of south sea pearls and a lot of photos of freshwater pearls from reputable vendors. After a while you will get a feel for it. However, you have to look at the big picture too. A high value pearl would usually not be set with poor quality diamonds.
Also, if the price is tempting, then maybe it really is too good to be true.