How would you know if your toddler has a speech delay?

My almost 3 year old had about 75 words at 24 months but didn't start combining them until around 28 months. Now, he makes 3-5 word sentences and has hundreds of words. The concern is sometimes when I hear him singing his nursery rhymes, he sounds strange. He is kind of sticking his tongue out as he sings. Should I be concerned?