If your child is not talking as much as other children their age, or they talk a lot but you can't understand anything they are saying, they would benefit from speech therapy. You do not need a doctor's referral to have an evaluation or therapy sessions. I have worked extensively with preschoolers who are not talking and have a very successful approach. I will also teach you as the parent how to help them at home.
Learn more here: American Speech/Language/Hearing Association