What is the best way to tell a 3.5 high-density disk from a 3.5 double-density disk?

The best way to differentiate a 3.5-inch high-density (HD) disk from a double-density (DD) disk is by checking the label or markings on the disk itself. HD disks typically have a capacity of 1.44 MB, while DD disks have a capacity of 720 KB. Additionally, you can look for the number of holes in the disk casing; HD disks usually have a notch and a second hole, while DD disks have only one notch. If available, using a disk drive that reads both types can also help identify their capacity when inserted.