You can guess... or you can you can find out for sure.
A water manometer will work fine, or you can buy a low cost Magnehelic differential pressure gauge from e-bay.
These come in a huge range of pressure ranges and scales, so be very sure of what you are buying.
Something that measures up to a "few" inches of water gauge pressure will be ideal for aerodynamic testing.
These all have two pressure ports, and measure the difference in pressure between the two ports.
The scales and pointers are quite large and easy to see while you drive.
With a couple of lengths of small bore hose, you can measure the air pressure difference between any two places on your car.
Where you can find the highest pressure difference either side of a panel, that is where you cut the hole to get maximum airflow.
Once you get one of these, and start actually measuring things, it will open up a whole new world of discovery.
About $70.00 brand new, or a fraction of that secondhand......http://www.dwyer-direct.com/shop/search ... HAodTQ-AXQ