Are you an iOS only user and have to sign up on ZwiftPower or use ZwiftGPS? Then you have to know your Zwift ID, but where can you find it?
Method 1: Easy and secure
A very easy way to find it is to go to my.zwift.com in Safari, pick any of your activities, and long press on the download link. The user ID is part of the URL to the .fit file, right after where it says ‘prod/’.
Method 2: On the web
Christian Wiedmann has provided a utility called What is my Zwift ID? at http://www.virtualonlinecycling.com/p/zwiftid.html?m=1
This utility looks up and displays your Zwift ID when you enter your Zwift username (e-mail address) and password.
If you change your Zwift password before and after using the script, it should be absolutely safe. As Christian says, it is safe even without that (the password is not stored anywhere), but of course you have to trust him on that. By the way, Christian is definitely trustworthy.