If someone comes to you to tell you. You have a 18 year old child what should you do.

Well really that would be up to you and the child now {adult}
If you believe its true and you don't want to get a paternity test
You could try and talk to him or her if her parents that raised the her is all right with it.
Then meet. If the child had no father all of those years she would probably benefit
By finally meeting you. More details would help in finding the right answer to your question.