Share 'Measuring fresh air intake ducted to a return and no way of measuring from outside.'

I am sure this is a challenge that some of you have faced before: You have an air handling system with fresh air intake that is ducted into the return and is terminated on the outside in a way that would not allow you to measure it with a flow pan.  My thinking is that there should be a way to measure the air flow by measuring pressure difference between outside and the fresh air duct and perhaps…