You kind of answered your own question :) If it shows up in the Scope for a Prestage Enrollment, then it is by definition a DEP assigned machine/device. They won't show up there unless your Jamf Pro console gets the information from Apple Business Manager or Apple School Manager that they are valid devices for assignment to a Prestage Enrollment. Also important to note is that the devices first must be assigned to your Jamf console within ABM/ASM before Jamf Pro will be able to see them in the Prestage scope. Essentially, if you see it showing up in the scope for a Prestage, then it's good to go. Hope that makes sense.
Note that a device can only be assigned to one Prestage, so if you have multiple ones and don't see a device show up in the scope tab that you're looking for, it may already be assigned (checked) for another Prestage.