The CFM will vary with the pressure drop across the rad, etc.
The Spal fans have a chart for this. I graphed several while making a call on which to use:
The blue line is the 14" fan I bought, the green is a 12" high output. At the pressure drop I measured (5.5 mmH20) the 14" pulls more.
Anyway, you'd have to find a way to measure the amount of air moved, but without changing the pressure behind the fans as that will affect the measurement. I've seen something similar done with building vents, and they use a calibrated box that pipes down to a 12" ish size tube that they have a MAF in. You'd have to pull an engine out to get enough space for the measurement rig.