One way is to create a Virtual Machine, and try installing Windows. In some point you will be asked for the key. See if it accepts Ultimate key. If it does, it is probably an Ultimate or Ultimate/Professional version. If it fails, try a Home Premium key. You can use Oracle's Virtualbox, VMWare's Player or whatever virtual machine application you want. If it is an 64-bit DVD you will need a 64-bit host system to install it. You cannot use a 32-bit host for a 64-bit guest.
Another way is to open in Notepad some specific files in the root folder of the DVD and check some specific details, but I don't remember now, so I cannot help you there. I would go for the first method. Once you find out, use a permanent marker to write it on the disk so you remember next time.