DNA testing on a blood sample, chest feathers or the egg shell that your bird hatched from can determine the gender of your bird with 100% accuracy. Surgical sexing is another method used to determine gender, but it's reserved for larger species of birds because the bird must be anaesthetized. Although not always accurate, your avian vet may be able to detect the gender of your bird by examining pelvic bones and feather patterns. Cockatiels have 2 bony ridges at the base of their abdomens, above the vent. In females, the structures are more flexible and further apart. A person inexperienced with breeding and examining birds should NOT try to do this. You can seriously injure your bird. Breeders frequently use visual clues which suggest whether a cockatiel is male or female. Most color mutations are monomorphic, meaning that there is no visual difference in the appearance of a male or female cockatiel. However, in normal grays, lutinos, fallows, silvers and cinnamons, males will have bright yellow faces and bright orange cheek patches. The female will have duller cheek patches, all gray or all brown faces, spots under wing feathers and barring on their tail feathers. You may need to hold one of these feather up to a bright light and use a magnifying glass to see them.

Female. The stripes underneath the tail feathers indicate they are female. Thats my guess anyway, I could be wrong

Most of the time female has a thinner head and are more agresive if not tamed. Males more calm than female u can also feel there bone at there butt and see if they Are seperated means its female most close to eachother is male. thats all i can say other way is dna testing.

sexing cockatiels - http://www.justcockatiels.net/sexing-coc...

They have both a tail like a hen however the one of the right has very bright colours and cheeks?