How do I find my old password that I set up this watch

If you forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage

Apple ID – Forgot Password  *      

Have you checked for the password used on the phone the watch is synced to?

Or do you mean the passcode for the watch? See the Apple Support article(s) below.

If you forgot your Apple Watch passcode - Apple Support