It is depend upon behavior, Males are often perching or patrolling in search of females, and females spend a lot time searching for plants on which to lay their eggs.
The male Large Skipper has a diagonal band of androconia on his forewings and the male Silver-washed Fritillary has four bands of androconia on his forewings.To tell if a monarch butterfly is a male or female, you need to look for a special spot. The special spot is called an alar spot. The sex of monarch larvae can be determined only in dissection. Males will have a testis located in the 6th abdominal segment, dorsal to the gut. If you have a last instar male caterpillar, the testis will appear as two bright red or pink sacs; often they appear to be one sac. Distinguishing between male and female butterflies is not a task that has to be left to a group of scientists. In fact, most butterflies have gender-specific characteristics that make sex identification rather easy. The size also differs but it depends on the species as to whether the male or the female is bigger. It depends on the species of butterfly the differences in species may be subtle-males generally have more angular wings, longer thinner bodies, brighter colours, and stronger patterns than females of the same species. The color pattern on the upper wing (dorsal) surfaces often differs in many species of butterflies. For example, in species of Blues, the male's dorsal wing color is often bright blue, whereas that of females has lots of brown. See these photographs of the Anna's Blue (Plebejus anna) . Another good way to tell the difference between male and female butterflies is by behavior. Males are often perching or patrolling in search of females, and females spend a lot time searching for plants on which to lay their eggs. The final and definitive way is to examine the abdomen. Females tend to have rounder abdomens. Males have a slit at the tip, and females have a round holes where the eggs are laid.
http://www.answers.com/Q/How_do_you_tell_the_difference_between_a_male_and_female_butterfly http://www.monarchwatch.org/biology/sexing.htm http://www.ehow.com/how_5811371_tell-between-male-female-butterfly.html http://www.answers.com/Q/How_do_you_tell_the_difference_between_a_male_and_female_butterfly http://www.kidsbutterfly.org/faq/appearance/7
how can you tell the difference in male and female butterflies