I see veins by about 5-6 days into incubation, but I have candled thousands of eggs, so I am used to seeing what I need to see. But before the veining is clearly visible, you should see the tint of the egg turning reddish color if the egg is fertile and developing (It's especially noticeable where the air cell meets the egg contents, at the big end of the egg). Most people wait until 10-14 days to candle Guinea eggs, because it's very obvious by then. Candling and comparing the differences in an egg from your fridge to a 10 day old incubated egg is like night and day at that point.