Clownfish live within social units. The female is the largest, most dominant clown within the group. The male is the second largest and the second most dominant clown within the group. The rest of the clowns remain adolescent (neuter) without the ability to reproduce.
Clownfish larvae undergo metamorphosis at about 3/8" in length. This is when they become juveniles and must acclimate to an anemone to avoid predation. If the juvenile finds a vacant anemone, then it will start it own reproductive unit after reaching sexual maturity. If the juvenile is unfortunate enough to settle into an anemone that is already occupied, it will take a role as the lowest clownfish in the unit. It will never reach sexual maturity until it is able to work its way up to male (usually the exisiting female must die for this to occur). Some clownfish will go through life without ever being able to reproduce because of this reason.
Size is a good indication of a female if two or more clowns are being kept in the same tank. However, size is not always accurate especially if looking at clownfish from different tanks.
I believe all clowns of the amphiprion genus will pair relatively easily. The Maroon clown (premnas genus) is a lot harder to pair. Just give it time. HTH