I'm pretty sure it's a girl because the lady at the pet store said she most likely was but it's so young it's hard to tell but it has bars or stripes on it's tail feathers. Please check out these pictures and tell me what you think.
http://s242.photobucket.com/albums/ff159/shelbski/?action=view&current=100_3152.jpg

cant see pic

with birds, you can tell from the dots on their "cheeks".
the fair colored/ pink.yellow ones are female
the red/blue colored dots are male.
i love birdies! raised cockatiels, parrots and parakeets for a long time.

It is recommended you wait until the bird is 9mos in order to have a more determined gender such as facial coloring and whether the wings have a solid color or dots (females have dots or little lines while males have a solid color). With the picture you provided I can only guesstimate that I concur with the pet shop associate on it being a female but it is extremely hard to tell. Mind you only using a facial method of reading a cockatiel is not conclusive you would want to look at the wings as well.

cockatiels you can't tell by their markings, you have to have a blood test done. It was $20. about 5 years ago.