How to check if a device is ready to be enrolled in our JSS via DEP

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.

Another way of checking is by going in to JSS settings>Global management>Device enrollment program then type in the serial # but what the gentleman above said is right.