A 1.44M drive will have a sensor on the right hand side (just behind the slot opening) to detect the extra hole found in 1.44M floppies. This sensor will be identical to the one on the left side for the write-protect hole. It may be an optical sensor, but more likely it's a mechanical switch with a plunger (1/8" tall, 1/16" dia) that can be pressed down.
Another sign that a drive is 1.44M: there are no jumpers to select the drive number or any other settings.
The connector pins are the same for 1.44M and 720K drives except for pin 2, the high/low density selection signal from the controller. All drives now instead use the extra hole in the floppy, and only very old controllers can even generate the signal.