How old is your cockatiel? Has he/she gone through the first molt or two? If it has, I'd say you have a female, but if it hasn't then it would be hard to tell if we can't get a pic as all babies have the dots and the lines.
Is your bird very vocal or quiet? Males tend to be more vocal.
Looks like you've got a pearl there. My guess is female. Males lose their pearling after their first molt or two. So if she's gone through her first molts and kept the pearling then you've got a young lady there.