It's not easy. If you have a lot of, say, 8mm bicones of both glass and crystal, you could weigh them (the glass separately from the crystal). The crystal should weigh more than the glass.
However, if you want to tell Swarovski crystal from non-Swarovski crystal, there are a few characteristics to check. First of all, all Swarovski beads will be cut the same, whereas non-Swarovski beads will vary in cut quality, style, and facet placement. Second, Swarovski colors saturation will be consistent throughout each color and each bead. Non-Swarovski colors may fade or separate within the bead. Third, drilled holes in Swarovski beads will be centered, consistent, and not overly large in each and every bead. In non-Swarovski beads, drilled holes may be on an angle/not true vertical, may be 1mm or more in diameter, may not be centered in every bead, and you might see more than one hole drilled in, say, an 8mm cube. Finally, in the case of an aurora borealis finish, the Swarovski beads will exhibit a subtle iridescence, but the non-Swarovski beads will exhibit an obvious, coated iridescent effect.