I, and including many FH hobbyists can't be 100% sure whether certain FH is fertile or not. The best way would be to try to breed it with a proven fertile female(When testing fertility on male) or try it with a proven fertile male(testing fertility of female), and find out! It might take more than one try for them to get it right, so don't assume they are infertile if no success at first try. The male just might not know how to fertilize the eggs and will eventually learn.
However, here's the general idea:
-Red colored eyes usually means they have high chance of being fertile.(ZZ,KML,ZZML,JK tend to have red eyes)
-White or orange colored eyes mean that they have less chance of being fertile.
(SRT,Kamfa)
-Female FHs with white or orange eyes are known to have higher fertility % rate than males, and this is because of they carry parrot genes in them. Most of you already know, 99.9% of male parrots are sterile, and a good percentage of female parrots are fertile. Kamfas and SRT have parrot gene in them, hence if they are males and pass on their father genes as a dominant gene, they will have higher change of being sterile.
I hope this makes sense and help those who are wanting to know!
Happy breeding!
JP