The character in question should have sufficient features to make an educated guess, such as visible primary/secondary sexual characteristics, like pussy/breasts. If there is nothing like that to go by/the character is truly androgynous/otherwise indeterminate, tag 1other instead. When there are clearly mixed characteristics present, judge by secondary sexual characteristics (what would the verdict be if the character's crotch was entirely hidden?). As such futanari characters are also typically to be tagged 1girl.
Though external context does not apply and only the submission itself in a vacuum is usually relevant, context from variant sets and sibling posts that clearly depict the exact same character can and should be considered. Even if only a hand is visible in one image of the set, hence would be tagged as 1other, but that character has visible breasts in another image of the set, tag all images containing that character 1girl accordingly.
Note that girl/boy/other tags don't apply when:
The character is only implied (talked about/pointed at but not visible/etc)
The character is depicted within a piece of media such as a painting, unless that depiction plays a major role
