How Do Birds Know The Gender of Their Own Kind?

For birds that can't accurately be sexed visually, such as rainbow lorikeets and albino cockatiels, how do those birds know each other's genders?
Can they immediately tell by looking at a bird or only if they perform courtship?
Answers:
They can't be accurately sexed visually by us, but likely can by them. They can see different colors in the spectrum, and many have markings we can't see. Also, I'm sure they're used to identifying individuals of their own species. After all, we're used to sexing people by their face and build, even when we all wear clothes. I'm sure there are subtle things we can't detect.

---

I'd guess it is a bird thing.

---

it is called pheremones! The males and females both secrete this oil from their body that makes them smell a certain way that sexually attracts the opposite sex! This goes for every kind of animal, even humans, but we don't know it when it happens!

---

I've always thought it's either some sort of scent we can't smell, or, since most parrots can see further into the spectrum than we can, that there IS a dimorphism that humans just can't see. Whether anyone else has thought of that and put parrots under black and infra lights to see, I don't know.
I know if you have a pair of sexed birds, the male usually does look a little different than the female--slightly larger, etc.
If I put two of my males together, they both start what I call the "horny dance" and when each sees the other isn't responsive and is instead dancing back, a fight will ensure. So maybe that little hissing, bowing, circling dance is how they tell.