Write the person a letter, requesting the new address, and send it to their "old" address. This letter will be forwarded to their new address, and if they respond to your request, you will have the forwarding address.
Wait until the forwarding expires, and mail a letter to the "old" address. For a certain period of time after the Post Office stops forwarding the mail, anything sent to the old address will be returned to sender, with a sticker that states the new, forwarded address. Remember to include your return address on the envelope.
Call them on the phone, if you have their phone number, and ask for the information. Chances are they will still have the same phone number, especially if it is a cell phone number.
Ask their neighbors at their old address. Often people leave their forwarding address with neighbors.