At three inches snout to vent (roughly 5.5" with tail) should be old enough to tell a male without even using a jeweler's loupe. (basically a fancy magnifying glass which fits in the eye. A 8x magnifying glass will work as well). But typically crested geckos when they are 6-9 months old the males have obvious hemipenial bulges past their vent.
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PHLdyPayne