The preflowers should give you a clear indication of the sex of the plant. Little hairs "pistils" are females, and tiny little stalks with what looks like balls are males.... pretty simple, You might see little "pistil-like" hairs growing on the sides of the stems, but these are NOT preflowers, preflowers always show at the point where leaf stem meets the main stem. Im sure someone has posted a decent guide with pics to determine sex somewhere on this site.
Good luck with the grow!