The penis just isn’t very sensitive in that way so it’s hard to tell. Usually can only tell that with fingers or that the woman says that certain positions are better or more fun for her.
The main vaginal difference you can discern with a penis is depth and tightness. I’ve had sex with more than few women where I was just too large for them (and I’m not that huge), so I don’t understand why guys are obsessed with having a massive schlong. And I’ve had sex with a woman who was a recent former gymnast who had much more control of her pelvic floor/vaginal muscles than anyone I’ve met before or since. She could deliberately squeeze my penis so hard it was almost painful. I’ve never been with anyone else like that, or with such fine control. The reason I mention it is that with her, I could tell what shape her vagina was simply because she could squeeze me hard enough when inside her that I could discern some shape — as I said, the penis just is not that sensitive in reality in that way.