When you ask for information on timing, location, and topic, add to your standard list: ask clearly and early which women will also be joining the panel. The more often you ask, the more often organizers will work to include women.
The audience will pick up on implicit, subtle cues that show disparity between women and male event panelists. Be aware of how you address your fellow participants and avoid distinctions, no matter your previous relationship. If you call the male panelist “Mr. Smith,” call your female colleague, “Ms. Smith,” even if you’ve known her for years.