How to tell double-density vs. high-density in 5.25" floppies?

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.