With experience. You use that to tell them apart...you look for certain traits in the nacre, their imperfections too...
But in many cases you do need to SEE the pearls, handle them to see the subtle differences and traits.
I do not know the vendor.
I would buy from a trusted, reputable vendor...now, for $140 dollars it could be a good price even for a freshwater pearl of the size and quality.
24K golden tone SSPs are hard to come by, so for me this is the giveaway.