The difference between an fertile Ostrich egg and infertile egg is as follows.
When we take the eggs out of the nests our workers clean them first. They then take the eggs to an darkened room where they use a normal torch-light to put against the shell. The eggshell becomes translucent and therefore a dark spot inside the egg will indicate an embryo. The ostrich eggshell is porous like your skin which means that if you put a torch against it you can actually see through the egg.
The infertile eggs will not have an embryo and will be used for decorative purposes like painted eggs, jewellery pieces, mosaic work etc. The general rule of thumb is that about 10% of all eggs laid will be infertile.