How do i check if the egg is good

Question
I herd if you put if under a flash light that will work but how explain and how long after the bird lays the egg should i check

Answer
I would recommend leaving the eggs alone for the most part.  The flashlight trick sometimes works, but generally unnecessary.  The eggs should hatch roughly 18 days after the female starts incubating (usually once she's had two or three).  If its been longer than 21 days or so, they're probably duds.  

If you absolutely can't resist a peek, here's the safest way.  When you check the nest (you should do this twice a day to remove any broken shells or dead chicks anyway), its best to do this while the female is stretching her wings or pooping, pick up the egg carefully, trying to keep it as level as it was when you found it.  Then, take it into a dark room and use a small flashlight (larger ones generally aren't focused enough) on one end of the egg. If it's fertile, you can see the chick in there after about a week.  If you look closely, you can even see its heart beating!  Replace the egg as soon as you're done checking, where you found it, and preferably not when mom's around unless you want a grumpy momma bird.