I've built a new PC and plan to purchase Coreldraw 2018 to install, but daftly I can't remember whether the X6 I already have is a full version or an upgrade. Is there any way of telling from the DVD?
Upgrade is a pricing thing. The software is the full version thus it's not upgrading what exists.
You don't need even to have X6 installed on your computer. The only difference is about the licence
The reason I'm wondering is that if mine isn't an upgrade then it means I can use it to qualify for the upgrade version of 2018, but the question is how to know.
go to corel.com and log in to your own user profile (top right corner), you'll find the information here
It doesn't matter if it's a full version or an upgrade, as long as its licensed you're eligible for the upgrade price.
From the Corel web-site: "To purchase the upgrade version, you must own a licensed version of CorelDRAW Graphics Suite. Please note that upgrade eligibility excludes Academic, Home & Student Suite, OEM and Not for Resale (NFR) versions."