A general rule of thumb is that kids who have no words other than "mama" or "dada" by 18 months should get a hearing test. I am a fan of hearing tests for all kids though--before they even leave the hospital for the first time.

By "words" we don't mean complete English words, but using the same sound for the same object, such as "ba" for ball or "u" for up. The first words kids learn are not those they hear most frequently, but those they hear most frequently as single word sentences, such as "no" or "bye-bye."

Using baby signs can help speed this process up by using the same hand signal or body language. The baby signs books are great, but you can also use simple ones that you come up with on your own. Also, kids who point at objects at this age will tend to gain language much quicker than those who don't. With no pointing or no significant words by 18 months, I send kids for a speech and/or developmental evaluation.