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.