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.
DD disks have a hub ring, HD do not. HD were quite common. There were actually Quad-Density disks also, which were a quirky standard used on some early Tandy/Radio Shack Model 2000 systems.
Notching is not an indication of single sided vs. double sided. A true double sided drive has 2 heads, and reads and writes both sides simultaneously. A Commodore, Atari, Apple, for example, only had one head, so people would notch the disk to flip it and use the opposite side. The media was certified for writing on both surfaces.
Many people warned that flipping the disks would lead to early failure, but I just used some 32 year old 'flippies' in an Apple IIe last night with very low failure rate, especially considering the age of the the magnetic surfaces.
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.
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).
The surface of DD disks are coated with iron oxide and when held up to strong light are brownish. (Think type I cassettes) HD are made with cobalt coating and are a blueish/purple tint when held up to the light. EHD are made with a barium ferrite coating. Ive never seen one in the wild, so apart from looking brownish, anything is possible.