Mostly you can't. Sexual dimorphism is the exception rather than the rule among butterflies (unlike birds.) So most species look pretty much alike and are difficult to tell apart. (Although they know, and that's what really matters.) Among sexually dimorphic species, the differences can be as subtle as the color of the antennae or as dramatic as looking like completely different species. Whatever the differences, they can be learned. And then it's easy to tell.
a male has a white dot on his head
You can't. You have to wait until it pupates. if it comes out of the cocoon as a hamster it was male, if a parrot it was female.
you don't
A male has a scrotal sac and a female does not.
You can find out whether your dwarf rabbit is male or female in the same ways that you can find out whether any pet rabbit is male or female -- dwarfs are no different from other rabbits in this sense. See the related question (linked below) for details and links.
that depents on which plant it is
Whether their opponent is male or female... Foot to the balls!
how do you know if your pygmy chameleon is a male or a female
It depends on the specific person. It doesn't matter whether they are male or female.
As far as I know, they don't however they are the deciders of whether the offspring is either a male, or a female.
Each fingerprint is different whether it be male or female, it does not matter.
Many people do not know whether or not los is a female or male pronoun in spanish. it is known that los is a plural mal pronoun due to spanish classes that people attend.
Depends on whether you are looking for male, female or she-male.
Whether male or female, the word pilot is used.
it depends on whether it is male or female ;)