It's very difficult to sex pieds. Unless they lay an egg or catch them masturbating/having sex your main option is a DNA test. Males will hump objects when masturbating, females will rub their vent while playing with their favorite toy. Females tend to masturbate a lot more just based on personal experience.
When choosing a baby Cockatiel, what are the best chances of getting a male? Our departed/much missed first was probably a male--he talked up a storm, learned words easily. I got him at 6 months. The breeder had him and others in a cage and said he was the noisiest! I greatly enjoyed him and teaching him words and would like to get another.