How is water resistance of a device measured?
You are right, the depth is essentially telling us the pressure the watch can withstand. Overall there are 2 tests performed, specifically on watches.


Dry test


In this test the watch is place in an airtight chamber and the pressure is increased. If there is any change in the dimensions of the watch then it is not resistant at that depth.

Once we determine a watch is not water resistant the next test is usually performed to determine at which point the watch is failing.


Wet test


The watch is placed in a chamber half filled with water and air. The pressure of the air is gradually increased and the watch is then immersed into the water. The pressure is now released. If bubbles come out the watch is not water resistant. The location of the bubbles is the fault location. (essentially what you do when you get a tyre puncture fixed).

You can easily perform these tests on your electronic equipment (in a protective casing), first doing the dry test. If you do not see any noticeable fault you can try the wet test. 

(If you are really ambitious i guess you could always pay a diver to take it to that depth!)

Reference:

http://www.prestigetime.com/page.php?water-resistance