The chances of all hard drives using the same grade of magnet is very low. Different designs call for different grades of magnets. It is possible that some hard drives use N52 magnets but in general there is no way to tell. If it is really important for you to know the grade, you can send magnets out to be tested. They would could use something like a DC Hysteresis Graph test to determine the magnet's properties.
Alternatively, you could get a rough idea by picking something up with the magnet and then comparing that to a known magnet. For example, if you have a magnet that you know is N52, you could try picking up increasingly heavy objects with the N52 magnet and your unknown magnet and see if they can pick up the same mass of object. If they both pick up a 3 kg object but can't pick up a 3.25 kg object, for example, you might be reasonably sure that your unknown magnet is a N52 magnet. If the unknown magnet can only pick up 1 kg, however, then you could be reasonably sure it isn't N52. This test would require both magnets to be the same size and shape.