How can my end user check if they are using the admin user for their QuickBooks Desktop connection?

Have your end user go through the following steps:

1. Open QuickBooks Desktop

2. From the top menu, select:
   Company → Set Up Users and Passwords → Set Up Users

3. Enter your password when prompted

4. If the User List shows "(logged on)" next to the "Admin" user, then that means they are logged in as the Admin of the instance

If this shows differently, then they are likely not on the Admin user. They will have to switch over to it to establish the QuickBooks Desktop connection or else the connection will not work properly.