Sorry, we really do need better photos. Larger, closer-up would help. Use a white background again and strive for the best focus you can get.
Please take a close up photo of any blemishes you see.
But you can also do some tests of your own.
1. Try the "tooth test": Rub one of the pearls in each strand gently against the edge of your front tooth, or else rub one pearl in each strand gently against another pearl in the same strand. Real nacre feels a bit gritty, while imitation pearls glide smoothly.
2. If you have a 10x loupe (jeweler's loupe, 10x magnifier) you can look at the surface of the pearls. Under 10x magnification, genuine pearls look very smooth while imitation pearl coating is a bit more coarse looking.
It's the opposite of the tooth test; real pearls look smooth but feel gritty; fake pearls look somewhat coarse but feel smooth.
If you are still in doubt you can compare what you see with a pearl that is known to be real, or known to be fake.
3. Round imitation pearl strands tend to be quite uniform in color, size, shape, and luster. There are also imitation graduated strands, however.
4. Look at the area near the drill holes.
• Lower quality imitation pearls may have swirls of excess pearl coating near some of the drill holes. However, better fakes won't have this.
• Cheaper imitation pearls may have beveled drill holes, or larger drill holes (and thus larger knots) than genuine pearls. (Real pearls are sold based on weight so the drill holes are small, to preserve weight.) Again, better fakes won't have this.
• See if there is any flaking of the coating. Imitation pearl coating is thinner than real nacre.