There's no way they wouldn't be able to tell. I saw this post a trans woman made, saying she went to a gynecologist and they said "I wouldn't have guessed you were trans!", as to imply that they couldn't tell apart her neovagina from a real one. If the story was true at all, the doctor probably said it before even inspecting her and she took it to twitter or whatever as proof that even doctors can't tell!.
I also saw this other trans woman tiktok, who claimed to have never disclosed she was trans to sexual partners (and encouraged people to do the same) and said no one was able to tell. She was very femenine looking, and at first glance I can say I wouldn't have thought it was a trans. One thing is the makeup, hair, voice training, fake titties, etc but a neovagina could NEVER fool anyone. That is pure delusion that every passing trans female propagates to others. I think they start lying to themselves and others so much to get the validation they're so desperate for that they become completely delusional about reality.