A certified letter requires a signature upon receiving. To send a certified letter, the sender must pay extra shipping to ensure the postmaster stops at the door to get the signature. It is often used for legal documents or mail that people want to ensure reaches its destination. This mail is often expected, but can come as a surprise. There are three ways to find out who the mail is from before signing for the item.
Ask your post man who the sender is before accepting the certified letter. There is no law saying they cannot tell you, and they have a tracking system used when the mail is sent. They can look this up on their pad or read to you from the return address.
Read the name of the sender from the peach card left in your letter box if the post man missed you. The card tells you who sent the letter, who it was for and a confirmation number. You then take the card to the post office to pick up the letter with an ID.
Ask the postman at the post office who sent the letter when you pick it up at the post office if the information was left blank on the card. They can look up the information on the tracking system or read to you from the envelope.