That is a tough question to answer. Very often training programs are structured so that overtraining is reached, and then backing off to allow the body to re-cupperate. I am sure people like CoolColJ and others can provide more input on this.
For me, I use common sense. If my strength keeps going up for about a month without slowing down, then obviously I am training in that optimal territory which is neither overtraining nor undertraining.