I would say you cant tell, and I agree a better predictor is te intelligent leve of their parents as if both parents are gifted the likihood is the child will be.
People aways assume we use something like that with DD since she reads at almost a second grade level, it is nice that she rsponds very loudly..."I taught myself!" Something i guess she is proud of LOL. she is very outspoken. I am interested to see where the kids who used this method are in several years as well.

DD6- DYS
Homeschooling on a remote island at the edge of the world.