how/where can i view an email sent using "hide my email address"?

See the article Use Hide My Email in Mail on iCloud.com - Apple Support

Note: In order to see Hide My Email messages in Mail on iCloud.com, set the forwarding email address to your iCloud Mail address or a custom domain address. See Change the forwarding address.

Use Hide My Email in Mail on iPhone - Apple Support

Those messages will be in sent folder .

See the section - How Hide My Email works

In the article What is Hide My Email? - Apple Support

Hide My Email generates unique, random email addresses that automatically forward to your personal inbox. Each address is unique to you. You can read and respond directly to emails sent to these addresses and your personal email address is kept private.