My Bird Had An Egg But I Don't Know If It's A Good Egg?

The mother or father are probably good indicator of whether or not something is wrong.  If they are no longer paying attention to the egg, not nesting it, etc.  They may already know it is bad.  As others said, try shining a light on it or holding it up to the light, you can then tell if something if forming.  Again, the parents clue are your biggest indicator.

First, is she sitting on the egg? If not, it is likely not good. If she is sitting, one or both may be feasible. You will need to wait about a week to be sure, but you can check them easily with a flashlight. MAKE SURE to wash your hands, better to use gloves, then pick up the egg (one at a time) and hold it up, shine the flashlight (in a dim to dark room) and shine the light through. If you can see a shape to the embryo you have a baby if not it is likely not fertile. I don't know what kind of bird so it could take longer than a week. As long as she is sitting on the eggs, you are probably fine. Birds will often toss an egg that is not fertile out of the nest. Birds also lay eggs whether they are fertile or not.... They have to go somewhere.

Try flashing it to the light bulb.