If you have access to a 10x loupe, look at the surface of these pearls close up. Real nacre looks smooth while imitation pearl coatings look coarse under 10x magnification. If in doubt about what you are seeing, compare with a known-real pearl or a known-fake pearl.
Conversely, when rubbed gently against the edge of a front tooth, or when 2 of the pearls are rubbed together, real pearls feel a bit gritty, while imitation pearls tend to glide smoothly (although if they are grimy, even fake pearls may not glide smoothly.)