According to Gang Task Force, some signs of gang membership are:
Poor academic progress in school.
Lack of interest in school activities.
Large blocks of unsupervised time.
Signs of increased conflict at home.
Frequent disciplinary problems at home and/or school.
Frequent contact with police.
Drawing graffiti.
Imitating gang dress or behavior.
Tattoos drawn on the body.
Uses hand signs adopted by a gang.[...]

Check out the full post on Robin's blog!