Being able to answer simple questions, ones that every girl learns by age 13.
Things like references to there 28 day cycle. I have commented that someone must be in the first 7 days of 28 and had the person in chat ask what I meant. When I replied "You know, your 28 day cycle?" I have been asked "What? I dont understand. 28 days of what?"
Most guys when asked their favorite color of lipstick reply with RED. Hint, there is no red. There is "Wild Orchid" and "Ruby" but no lipstick is just RED
You can also ask about pantyhose sizes, panty sizes, dress sizes, Hair colors, etc, etc. Lots of things that guys pay no attention to.
Then you get to the more obscure stuff like sex during a period tends to relieve cramps, Women will eat something before a first dinner date because the date is about the conversation and not the food. Heck if she gets into Sexting you can really toss some fun stuff in like descriptions of what you are doing to her labia or how you are pressing her cervix. Then watch to see how they reply, do they even know where those parts are?
The joy of growing up with 2 sisters, having two daughters, a wife, and a girlfriend. You learn a lot about women. lol