Fertile Corydoras eggs tend to go a tan sort of colour. After a few days you can just about make out the embryo developing inside (if you screw up your eyes/use a magnifying glass)
Infertile eggs tend to go/remain white and have fuzz growing on them.. You can see four dead ones in the group below.
I tend to just let nature take its course but I'm sure that, with a steady hand, it will be possible to separate the good eggs from any bad ones without damaging the good ones.
The eggs in the pics above are from C. paleatus, but the story is much the same for C. aeneus.
Hope that helps,