You can check this by using a battery to create the negative voltage: put the battery's positive to ground and negative to the Vo pin. You will see very dark displayed on the lcd.
He is already seeing every pixel very dark which leads me to believe that he doesn't need a negative voltage (w/r to GND) on his display. Connecting the battery as you say won't hurt anything so it's worth a try.
Just to clarify things, you normally connect the ends of your potentiometer to the positive supply (almost always pin2) and GND (almost always pin 1) and connect the wiper to the contrast (pin3).
Until fairly recently most LCDs ran with a +5 V positive supply. Since the contrast voltage should typically be about 4 to 4.5 volts less than the supply voltage this will place the contrast pin between 0.5 and 1 volt above GND.
If your LCD is running from +3.3 V then you may require a negative voltage (w/r to GND) at pin 3. To do this you leave everything connected as described above except for one end of the potentiometer. You disconnect the end of the potentiometer from the GND (pin 1) and connect it instead to the negative end of your battery. You connect the positive end of your battery to GND.
You should see all of the pixels very dark with the potentiometer at one end and you should see a blank display with the potentiometer at the other end. On your 4-row display you should see dim blocks in the first and third row but the second and fourth row should be blank when the contrast is properly adjusted. Don't try to run your sketch until you can get this action, you will be wasting your time.
Don