"I would say if you see yellowing on the fan leaves or the cotyledon leaves yellow up and die then give them a small shot of the grow big if they haven't shown signs of flowering yet. If they have, give them bloom nutes but make sure there is a bit of N in it to keep them green. And as always, start low. "
could not have said it any better. Another tip I would ad is to keep the lumens on the low side before they show sex. If you give em too much light in the early stages they tend to be 12 inch stumpy plants that are all bud. If you avoid over-lighting them they grow into a 24 inch bush. I use 250w HPS and just keep it about 14 inches above the top of the plant until they show sex then lower them and the plants explode. That's how you grow a big lowryder2. Oh, and any insult before they show sex, like Nute burn will make em stumpy, so don't ad nutes until they show sex or start to yellow like he said.