An IRS audit notice, or Notice of Examination, will arrive via Certified Mail and will contain the following information in the top right corner: your name, taxpayer ID, form or notice number, tax year(s) under examination, IRS employee ID number, and IRS contact information. The letter will also contain details of what the IRS is examining, which documents you need to send, how to respond, and the deadline to respond.
Here’s an example of an IRS audit notice (not that your IRS audit letter may not look exactly like this):

An increasingly common audit notice is IRS Notice CP2000. You may receive this notice if you have a source of income that was reported to the IRS by a third party (such as a cryptocurrency exchange or brokerage), but that you didn’t include on your tax return. A full audit is often required in order to dispute the balance due.