This is a good topic. While there isn't really a totally standard way to do this testing, the most common way is to run steady-state rpm and torque/load points with a loading dyno. An engine dyno is easier but that's not an option here.
As airflow increases, the localized pressure will drop. You'll see this on turbo cars especially--it's important for calculating pressure ratio on a compressor map. When you get a heavily boosted engine the restrictions get crazy and that's when rubber pipes on turbo inlets start to collapse and such. You can use different instruments and different measurements, like using a pitot tube instead of a manometer, or doing static pressure only vs taking into account dynamic pressure.
What you'll end up with though is a chart like this:
So x axis might in this case be 25 grams/sec increment. Y axis would be in 1 kPa increments for something like this. Each point is a steady state test from holding the dyno at specific torque points to capture different airflow. Then through the magic of Excel you put a curve fit in there. So at 100 grams/sec you could have atmospheric pressure be 98 kPa, measured pressure be 96.5, and your pressure drop would be 1.5 kPa.
There's always an exponential relationship between mass airflow and pressure drop--when you change the intake pipe you change the shape of that line through the points. Then you can start comparing across different intake pipes or even across different engines if the measuring process is similar. Once you collect enough data trends you can place your intake within an upper and lower boundary line to see how good the design is.