As the parent, you are not responsible for making the determination. The therapist should explain to you what goals and objectives are being set for your son. Depending on the child's age, he may be involved in the discussion as well.
Your son's speech therapist should work with you to determine what the goals and objectives are. I'm guessing that the therapist will work with your son to try to get him to understand the meaning of words through reading and pointing out and naming objects. But I'm not sure how you determine when the goal was really met.
You need to talk to the speech therapy person, they can answer your questions on his therapy. You need to ask questions if you have them to the person taking care of his therapy.
