You should be able to tell right away. Initially there is a spot in the yolk (the yellow stuff), then the veins and things come after that (probably within a week or so).

If you don't want baby cockatiels, there is a website where you can purchase fake eggs that she will sit on as if they were her own. I bought some for my parakeets and they worked beautifully. In fact, one of my females is sitting on some right now. www.theEggShop.com