How soon???

I have a Blue Marble thats paired with a very handsome (In my opinion) smokey, that I think has other genes hidden (He has swirly bits in his dorsal for instance), if he should happen to have a blue gene as well (he is quite colourful), I understand some of their fry could be blues, so how old will they be when I should spot if I've got lucky?

I''l try and get a picture of him shortly

A lot of people talk about a "blue headlamp" which can be seen on fry from a few weeks old, personally I'm not totally convinced, I've seen something similar on fry that don't have the Philippine Blue gene, whilst it's not always obvious on fish that are blues. I think a lot depends on the lighting and the angle the angels are viewed from.
I have a batch which are currently 5 weeks old (from when they went free swimming) where half are blues & half aren't, at this stage there are fish that I can be confident are blues, but only because I know for sure that some are. A second batch which are about 3 1/2 weeks free swimming are all blues, but I don't think I'd be able to identify them as blues if I didn't know the parents. Some varieties are easier than others, I find that paraibas are the most difficult, I have a mixed batch where both parents were het. for pb which are over 7 months old, whilst I was able to pick out blues from their non blushing siblings with 100% confidence from about 3 or 4 months old, there are some of the blushers that even now I'm not 100% certain if they're paraibas or not! although there are others that I've been certain about for at least a couple of months.

Ok thanks Rob, I'll keep looking