Measuring the aero pressures for yourself is always far better than reading theory. It opens your eyes in a way that nothing else can.
So, the measured panel pressures on my Honda Insight, equipped with effective undertrays to develop low pressure under the car:
Then, looking only at lift/downforce, the vectors (arrows that show magnitude and direction of resulting forces):
Then a diagram that shows (by squiggly areas) the rough integration of force x area. (To get better accuracy you would take more measurements, and also split the forces into their X and Y axes, but this gives you the general idea):
If you look at the squiggly lines, you can see a greater area of squiggles under the car than above ie this car heavily modified car develops downforce (don't forget to include the downwards pressure in front of the windscreen).
These measurements can be done for any car, on the road, easily and pretty cheaply (under US$100 for the tool kit that you can use forever).