I wouldn't even consider short hair as being necessary to be a tomboy. If that were the case, then Revy from Black Lagoon wouldn't count, and she's definitely a tomboy.

Your general approach in identifying someone as a tomboy mainly should boil down to their personality and attitude alongside their hobbies.

Being a tomboy is generally doing things that society perceives as a "Male-focused activity" or "Personality Traits that is almost always associated with Males". Clothing and Physical appearance matter little in determining if a Female is a tomboy, because a true tomboy is someone who is perceived by society as "Boyish" down to the person's very core.

You can call a muscle girl who dresses like a man a person with a tomboy aesthetic, but a real tomboy is a female that is seen by society that is breaking the norms by enjoying things that males like wholeheartedly and actively participates in hobbies and activities, as well as having characteristics that are deemed "Manly".