a question on cockatiels and eggs.

My tiel is about 3 years old and she has laid 4 eggs within 2 weeks..she just started sitting on them yesterday...and she has sat on them all night and day so far--only got up to eat and drink..
My question is how long does it take to be able to use the flash light trick to tell if they'll hatch into babies???
I have had baby tiels hatch before...and I remember when you hold a light under them if there is little red lines/veins then that usually means there's a bbaby inside..but if there's only yellow stuff then that means it's just an egg..so how long til you can tell w/a flashlight?? I seriously forgot...thanks if anyone remembers!! ;)