Export your total list of devices from the Apple DEP/Apple Business Manager/Apple School Manager portal. It will have all the serials of your DEP devices.
Export an advanced search of all computers and of all mobile devices with the display set to show serial number so it shows up in the csv export.
Open all the exports in Excel, select the serial columns, highlight/color cells of duplicates. Non highlighted serials are your non-dep devices.
Slight variation on @chriscollins theme.
If you have all valid devices in a prestage enrollment you may be able to get the first piece of information by exporting the devices in the prestage section in Casper, then compare it to the list of all devices actually enrolled in Casper, the difference will be the none DEP devices.