And if you want to see how much lift your fastback shape is developing, it's pretty easy - and accurate too. What you do is to measure ride height - and then see how much it increases at speed.
I first did this back in 2008 using the ride height potentiometers fitted as standard to my car (a Lexus LS400). They are used in that car to auto-level the HID headlights.
To dial-out the variation in ride height caused by suspension deflections over bumps, I used a Fluke Scopemeter to monitor the potentiometer output, doing firstly the front and then the back. The Scopemeter has an averaging function, so you could set it to zero, then drive (say) a few kilometres on a freeway and see the average ride height. Note that value, then do the same, but at a faster or slower speed, and then see the difference.
These days, lots of multimeters have an averaging function that works with quite high speed sampling - eg my Fluke 287 can do this. It's also possible to build a very simple circuit (a resistor and capacitor) and smooth the voltage output of the pot, allowing height to be monitored with any multimeter.
On my Insight I use the suspension height potentiometers (ex Range Rover) that I have fitted as a major input into the air suspension control system (eg I automatically lower the car at speed). I also take the output of these sensors (two front, one back) and feed them into my MoTeC dash. I then apply a smoothing (averaging) function to the display so that I can see ride height 'live', averaged over about 5 seconds on my dash.
Another way to measure ride height is to use an ultrasonic sensor looking down at the road. I have experimented with such a system, using an Arduino and XOD code that I wrote to display measured height. This worked - kind of - and showed that my wife's W212 Mercedes has a fair amount of rear lift. (That matches my driving experience of the car.) However, the ultrasonic system is accurate only to about 5mm and so I am not totally happy with it. I have bought a LIDAR sensor but I am yet to do anything with it.
Once you can measure how much the suspension height changes with speed, it's easy to turn it into actual kg of lift or downforce. For example, if the front ride height increases by 5mm average at 100 km/h, all you do is replicate that height increase with the car stationary, eg by pulling the front upwards via spring balance (crane scale, etc). The amount showing on the scale is the lift generated.
On the Insight, I measured downforce in this way by loading the front axle line (and then the rear) with bags of sand until the deflection downwards matched what I experienced on the road. (When doing this also bounce the suspension to take out damper stiction.) This approach also takes into account varying motion ratios in the suspension. You can measure at two different speeds and ensure that the measured force has gone up as the square of the speed.