If you don't see the fax number or upload instructions in the body of the letter, check the upper right hand corner. Sometimes the "contact us" information is there. If a fax number isn't there, a phone number and possibly a case number, at least, should be so that you can reach out and ask for additional information.
Is there a notice "type" number in the upper right corner? Like CP2000 or LTR3064C or something similar? Those will usually help you to identify real letters from the IRS.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
