My cockatiel laid eggs we recently pair her up with a male,I just want to know how can I be sure they are fertile... ii don't really know who laid the eggs becaouse the bird I thought was a male has been pooping really big.. please help

I've bred cockatiels in the past - I never disturbed the eggs, just left them and they either hatched or they didn't.

Sometimes if you handle them you damage them. I know you can candle them if you know what you're doing but I never bothered.

Girl birds poop straight down. Boys sort of poop at an angle. I think girls make bigger poops in the morning.

To test for fertility you can either

Take them to a dark room after 1 week of incubation and shine a bright light into them this should then be visible to all as veins will be growing inside the egg and should be easy to see,

Or you can use the water test method, this you need water heated to 37 degree Celsius , a thermometer to make sure the temp of the water doesn't drop to low if it drops below 35 degree c then remove the egg

Carefully place the egg into the water and if it sinks it is infertile if it floats it is fertile however all because an egg is fertile doesn't mean that anything will hatch from it ,
This method has a very interesting observational purpose also

In my observations using the water test method I have found that you can tell give or take a couple o days when the eggs will hatch by how it sits in the water, you can tell at certain stages after so many days dependent on egg sort weather the chick is alive or dead, and when it has done its internal pip,

It will take around 18 days for the first egg to hatch and usually you may ge a couple hatch togather then every other day 1 will hatch after that...
Could go on forever about eggs :p one of my favourite subjects but won't bore you all with it ,

Anyway be careful not to touch the egg with a bright torch as the bulbs get hot and can damage it and if using the water method then make sure when you add the egg to the water that you do it very gently as if put in too rough it may hit the bottom of the container with the water in and crack the egg,

Best of luck with the eggs
Katie