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.
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