I recently found a box of old 5.25" floppies. Is there a way to tell the double-density (360K) 5.25" ones from the high-density (1.2M) ones when the disks do not have any markings? It looks like the DD ones have the hub ring in the centre (if you’ve seen one you know what I mean) and the HD ones don’t, but maybe I’m mistaken.
Unless the manufacturer chose to mark the disks, there's no difference between them.
The ring in the centre of the disk was a manufacturing feature to prolong the life of the disk itself.
Double-sided disks will have the write-protect notch on both the left and right sides of the disk if they were intended for use in single-sided drives, but even that isn't a reliable indicator of sidedness.
You should generally be safe assuming DSDD for 5.25" disks, however, as DSHD 5.25" disks weren't very common (3.5" disks were more durable and reliable).
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This site seems to confirm my observations:
DD 5.25" (360kB) diskettes look very much like HD 5.25" (1.2MB) diskettes; however, HD diskettes seem to almost never have a hub ring, while DD diskettes usually do. The hub ring may be white paper, etc. and easy to spot or cut from the same material as the diskette and barely noticeable. This difference has been mentioned by Peter Norton (of Norton Utilities fame) in one of his books.