I was given presents. I have now discovered that the items are stolen property.

Answer

If you are aware of the owner of the goods given to you, then it is Waajib to return it to him/her/them. If you are not aware, then ask the one who gave you the items, who the owner is. If this is not possible or if he too does not know or he refuses to provide the information, then it is Waajib to give the items to the poor. It is not permissible to throw them away.