Put the disk into an appropriate drive, then use the following command: dd if=/dev/fd0u1680 of=/dev/null

This assumes that a 1.68M disk is in the first floppy drive; adjust and correct the device file as necessary. If the disk reads correctly, then the output should be (in this case):

3360+0 records in
3360+0 records out

If the two numbers do not match, or the system generates errors, then the disk is at best unreliable — use another. The number of records reported should match the appropriate number of blocks listed in the previous table.