You might want to give them a bath in warm sudsy water (use soap, not detergent, and bottled or filtered water.) They are quite grimy and that can make it hard to see what they are. Wipe them clean with a soft cloth.
I have a strand of genuine akoya pearls that I bought from a pile of costume jewelry. They were so grimy that they had no luster until cleaned up, and were thought to be fake. It's worth taking a little time to clean them.
When they are clean, rub 2 pearls from the same strand together gently without applying pressure. Imitation pearls will tend to glide smoothly against each other, whereas real nacre is a bit resistant. But grime can make them resistant too, so they need to be clean.
Usually imitation strands won't have gold clasps nor will real strands have base metal clasps, but the clasps may not be original to the strands. People sometimes replace clasps when restringing.
Look at the drill holes. Imitation pearls may have some of these characteristics:
• beveled drill holes
• swirls of excess pearly coating
• wide holes (real pearls are sold by weight and so tend to have very small drill holes)
• peeling coating with a glass bead visible underneath (genuine pearls do not have glass beads; they would have a mother of pearl bead.)
Look for other areas of peeling coating. The pearl next to the clasp in photo #6 looks like it may be peeling. This would indicate an imitation pearl. Real nacre can peel too, if it is very thin, but it won't look the same as peeling imitation coating.
All of that said, GemGeek is right about the value of pre-owned strands. They do not seem to bring in much money. And you would have to deduct the cost of restringing-- so there would not be a lot of profit.
Still, you may wish to restring them to wear them if they turn out to be real, especially if they have any sentimental value to you.
