discharge

Use the outlet pressure gauge and the blower performance curve.

One method is to do a ducting traverse with a pitot-static tube (they even have digital ones these days so you don't need to even bother with calculation).

For an accurate number and with a round duct you should take at least 20 readings along two diameters - a rectangular duct between 16 and 64 readings. The duct diameter should be at least 30 times that of your pitot. Locate the pitot tube section providing 8-1/2 or more duct diameters upstream and 1-1/2 or more diameters down stream of Pitot tube free of elbows, size changes or obstructions; and if possible provide an egg-crate type of flow straightener 5 duct diameters upstream of Pitot tube.

A rougher estimate could be taken with a single pitot-tube reading in the center of the duct and factoring with 0.9 to obtain an average.

If you don't have this capability - I am sure a local HVAC contractor would be more than happy to help you out.

http://www.imeko.org/publications/wc-2003/PWC-2003-TC9-004.pdf

What are you interested? Discharge flow or discharge pressure?