Knowing who is who to me involves the cultural environment you place your characters in and how willing they are to reveal themselves. If the cultural environment is conservative and sexually repressed, most likely you will know very few queer characters because they will do their best to stay hidden and be out only among those they think they can trust. But in a more sexually liberated culture, you might know a lot more queer characters depending upon the degree in which they out themselves.

In the 1970s I didn't know any other queer people, but there were people I suspected of being gay. To be openly gay in the south at that time meant ridicule and ran the risk of being targeted for violence.

But now in the very same area many people are out as both transexual and gay. They still run the risk of ridicule and perhaps violence, but that is rare and government officials will not arrest you for sodomy laws and you can be married. But still many people still feel uneasy about being out to everyone.

I live in a very liberal area of the country. I am open to my close friends, some coworkers, and only certain family members. Coming out to someone is a never ending process and it always comes with the risk of rejection. I think a novel should reflect the environment vs the openness of each individual character. I doubt that in any cultural climate you will ever know the sexual identities of the majority of people you meet.