All clown anemones have unique sexes. It appears that the females occur less frequently in the wild than the males, but there hasn't really been an extensive study across species and geographies. Best way to tell sexes is to get them to spawn in your reef 
however in some cases (like S. gigantea) you can tell the females because you can see the eggs with strong back-lighting.
All clown anemones reproduce sexually. It is likely they are all synchronous spawners - that a few males begin to spawn and presence of sperm in the water triggers other males to spawn as well as the females. Mass spawnings like this are not unusual on the reef with corals and other inverts - and some reefers with multiple anemones in the same system have witnessed this behavior. Initial spawning appears to be triggered by a combination of environmental factors, including water temperature, tides, and lunar cycle.
SOME clown anemones reproduce asexually as well - the most common being E. quadricolor (BTA) and H. magnifica. Asexual reproduction with other species has been witnessed far less frequently - if at all.
I'm keeping my comments general because there are some differences between species, and for some species (S. gigantea, S. mertensii, S. haddoni) there is very little information available.
I've got some great papers on anemone reproduction if people want. Dr. Anna Scott has been doing extensive research down in Australia on sexual reproduction of H. crispa and E. quadricolor and she has successfully raised offspring to adulthood. It doesn't appear terribly difficult - but you have to have the proper setup for it. It certainly appears less difficult (to me) than raising clownfish. Hopefully one day I'll try it with S. gigantea
however in some cases (like S. gigantea) you can tell the females because you can see the eggs with strong back-lighting.All clown anemones reproduce sexually. It is likely they are all synchronous spawners - that a few males begin to spawn and presence of sperm in the water triggers other males to spawn as well as the females. Mass spawnings like this are not unusual on the reef with corals and other inverts - and some reefers with multiple anemones in the same system have witnessed this behavior. Initial spawning appears to be triggered by a combination of environmental factors, including water temperature, tides, and lunar cycle.
SOME clown anemones reproduce asexually as well - the most common being E. quadricolor (BTA) and H. magnifica. Asexual reproduction with other species has been witnessed far less frequently - if at all.
I'm keeping my comments general because there are some differences between species, and for some species (S. gigantea, S. mertensii, S. haddoni) there is very little information available.
I've got some great papers on anemone reproduction if people want. Dr. Anna Scott has been doing extensive research down in Australia on sexual reproduction of H. crispa and E. quadricolor and she has successfully raised offspring to adulthood. It doesn't appear terribly difficult - but you have to have the proper setup for it. It certainly appears less difficult (to me) than raising clownfish. Hopefully one day I'll try it with S. gigantea
