Male and female cockatiels look similar during their early stages as chicks. Until their plumage change during their adulthood, you can just guess the probabilities only. When the cockatiels are round a year old, the bird will molt for the first time and grow a new coat of plumage, typically more colorful and with more variation between the sexes.
yellow head and orange cheek circles. 

Male 

• Males have dark and bright Yellow head and orange cheek circles.  Not applicable for albinos, lutinos and white face Cockatiels.
• Males have plain colour tail feathers
• ​Males will have long crest feathers
• ​Males whistle to attract the females
• Males show up courtship behavior by dancing and hopping

Female ​

• Pale or dull head and dull-orange cheek circles. Not applicable for albinos, lutinos and white face Cockatiels.​
• Females have bar/line/dot patterns in the tail
• Females will have comparatively short crest feathers than males
• Females don't whistle
• ​Females lay down and makes a small murmuring sound and accepts the male for mating.