Females and males generally don't look any different in terms of shape.
They do, it's easy to see in mature percula and ocellaris clowns. The females are deeper bodied and have more bulk on the top whereas males are more slender and look longer bodied. Other clown species may look nearly identical but an experienced breeder can normally tell in mature specimens.
Have a look at wild pairs and it's easy to see
False clown anemonefish (Amphiprion ocellaris) pair, female with smaller male swimming past sea anemones, Misool, Raja Ampat, West Papua, Indonesia Stock Photo - Alamy
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