You can decipher seedling vs. vegging state by watching the plants. For the first 2 or 3 weeks in dirt
there is very little vertical growth as the plant is busy establishing a root system. This is the seedling stage.
All of a sudden you see they are growing an inch a day and filling out. That's your veg stage.
I just keep track of "days in dirt". I've found that 31-35 days in dirt will find most plants around 26-30 inches
tall, just where I want them when I switch the lights. If you only have one strain going it is easy to decide
when to switch the lights. Veg until you start to see the signs the plant is showing sex. You'll see little
growths where leaf stems meet the main stem. That's your sign to go to 12/12 as the plant is ready to flower.
Good luck, BigSteve.