I could likely tell with a close up. can usually discern after the 4th node arrives. you will see the preflowers before switching to 12/12......usually
preflowers typically show in veg, I see them from weeks 3-4 from seed. Look at the node, you'll see little white hairs coming out, you can't miss them. Once you see it, you will know what it is. Just study the nodes every day and you will see it, you can't miss it, especially since you know what it looks like without preflowers.