An "old school" technique is to look at trichrome structure. It starts as a cone that forms a ball on top. The ball collapses, looking like a mushroom cap, and eventually falls off. When most of the trichromes are at the "mushroom looking phase", the plant is ready for harvest. 

 Yeah, I normally look at color. But this is also a valid technique.  

 Nearly everyone looks at color now. 

 I do a mix of the two because sometimes that mushroom thing will happen before the color turns. 

 Maybe different strains ripen different ways? I remember a sativa (Juicy Fruit) that NEVER ripened, and that was an indoor garden. After four+ months in the flower room, the owner harvested anyway, since it was messing up his supply vs legality status for medical.