Also refer to: <a rel="nofollow" target="_blank" href="https://www.usa.gov/irs-scams">https://www.usa.gov/irs-scams</a>
When you go to the IRS web site, you should see a padlock icon in the address window of your web browser, and if you click it, it will give you details of the security certificate. This means the site is verified by an independent trustee and is the real site.
If you don't see the padlock icon, something is wrong and you should not enter any personal information.
This is a legitimate site, but it is still possible for scammers to send fake letters asking you to go to a fake site that is one letter different (for example). So you always need to be careful.