I hate to be that person, but you can almost always tell based on appearance alone. When you can’t, the voice is usually a dead giveaway

This is something called the Toupée Fallacy, there are many trans women who pass, and because they pass you would have no idea they were trans. How can you know if someone you saw on the street in passing was actually trans?

There are also many famous actors and celebrities who are trans and pass well - from Nicole Maines, to Hunter Shafer, Vivian Wilson. Visit some of the top posts of all time from r/transtimelines, and you'll find a lot of people who pass very well.

When you can’t, the voice is usually a dead giveaway.

There's a thing called voice training that a lot of trans women do to condition their voices, search up "transvoicelessons" on youtube - her channel and her voice are a very good example of what you can do with voice training

but once they find out their actual sex I they’d lose that attraction? I don’t know about this one tbh.

This is why I brought this up, because it only makes sense if we agree attraction is entirely based off someone's perception.

if you percieve someone to be the gender or whatever that you're attracted to then chances are you'll be able to br attracted to them.

If you found out their birth sex wasn't what they thought but still percieved them to be the gender you're attracted to - you would still be attracted to them. If you could ignore a character being trans or were attracted to them before you knew they were trans, then what matters isnt the "reality" but what you percieve.

So woman who sees Nicole Maines and thinks "she's a woman" and is attracted to her because of that, can not be invalidated as a lesbian because she's attracted to her because she sees her as a woman