Looks like Black, Dythemis nigrescens, but I am not positive.

As for sexing, usually you can tell by the shape and size of the cerci, at the end of the abdomen. In females, the cerci are basically non-functional, so they are often shorter, straighter, or less "shapely" than those of the males, which need to perform a function in mating. There are other ways which vary by group - in clubtails, for instance, the male genitalia are often prominent under the basal abdominal segments, and in darners, the shape of the hindwings differs between sexes. Also, females generally have a thicker abdomen, but that can be difficult to tell with slender dragons like the setwings. With Skimmers (like these), look for the male cerci to have some shape to them, and usually converge towards the tips, whereas the female cerci will usually be straight, cylindrical, and parallel.