Without using or building a fan dyno its hard to get precise numbers. However, to get you into the ball park, current consumption is one indicator. Fan manufacturers (good ones anyway) rate their fans based on the restriction they see in the application, this is usually a chart or table with CFM's versus static pressure ( mm or inches of water). A example from a reputable OEM manufacturer is shown here where thye are kind enough to include current draw as well. The air flow are in SI units or cubic meters per hour but you get the picture.
In ball park terms , no fan will see zero restriction (0 static pressure) when mounted on a rad. in an automobile.
But cfms are near proportional to current draw in amps (roughly speaking). If you measure about 10 amps at the fan, you are close to 1,000 cfm, 20 amps; 2,000 cfm and so on. Its easy to measure current draw at the fan with a good auto grade current meter. Almost forgot, make sure you know the test voltage, most fans should be checked at alternator output voltage or 13V as a standard, easy to do when engine is running. Your wiring/relay must be top notch to support a good fan. Good luck in your quest and when in doubt, use a high quality fan with known performance, your engine is just too expensive not to.