Try Googling "Marijuana preflowers" and select images.. you'll have more free info than the law allows. Also, in addition to JC's Medical Growers Bible, which has plenty of pre-flowering info/pics, there was a great article in HT Beginners Guide (collectors edition..) on sexing plants..

Preflowers generally occur about the 4th week of vegetative growth, or when the plant is 6-8 weeks from seed.

Peace, Farmer Rich