tokay

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You *should* be able to tell just by looking at it. When it reaches sub-adult size, males will start to show bulges behind their cloaca. With some males, its pretty obvious (think - nuts).
 
You can also look at the preanal pores which are located in a "v" shape on the body side of the cloaca. Males will generally have more pronounced ones with what looks like wax coming out of them, whereas females will just have tiny holes.

EDIT: Here's a link with pictures: http://www.igniche.com/tokay_sexing.htm
 
MonsterFishKeepers.com