Hey there....
I look at the structure of the seedlings, the length, width of the stem... What kind of leafs they have - long, wide, narrow, fat etc.
How the leafs responds to a higher light-level or responds to nutrients.
Im looking at them like they were adults - full grown plants but in miniature.
As I have seen a couple of plants been grown from start to finish I have tried to - or, I just remember details from them that is important for me to know to get big buds.
You as a breeder must have seen 10.000s of plants been grown from seed to weed and you could probably pick a winning seedling at day 1.... or a 1 that got the traits that you are looking for
I know the environment plays a big role in how the girl/girls will grow and IMO the biggest factor is how the grower adjusting their lights throughout the grow, but especially in
the early stages (say first 3 weeks) of growth to get that perfect start.
I want to be clear that this is only my second attempt in hydro so you are more then welcomed to leave a tip or two