The sure way is to buy two or more juveniles and raise them together. The following is a non scientific account of juvenile clown fish maturing to adulthood.
Juvenile clowns are born males (some authorities claim, non sexual). As the group matures the most aggressive juvi grows larger and matures into a female. The next most aggressive juvi matures into a sexually active male. The rest remain sexually inactive unless they wander from home and pair off separately.
Not knowing the history of your clown, it would be impossible to determine the sex without doing a biopsy. So, your best bet if you want to pair it up is to buy a small (juvenile) Maroon and place them together. Be prepared to separate them. Your larger, older clown may not take kindly to a like tank mate. If this happens, I've had success partitioning off the two and letting them "get used to each other" through the glass for a few days or weeks. What happens after that is anyone's guess.
The problem is, not knowing the sex of your fish and not really knowing the sex of the fish you want to introduce it to. If, for instance, yours has turned into a female and you inadvertently introduce another female (after all you really can't tell the difference), neither of them will turn back into a male. Once turned, done.
It's still worth a try, if that's what you want to do. Just be prepared to find a different home for one of them if things don't work out.
Dick
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