How to find static pressure in duct system during commissioning?

Sounds like its too late but should have done Energy Star certification with a Manual J. A mechanical engineer who does residential or commercial forced air systems should be able to help you out.

You need to have a pressure/vacuum gauge and a pitot set to do this. Typically you will drill a small 1/8" hole in the supply side and return side ductwork, insert the pitot and measure the pressures. One will be positive and one negative. Then you add the two of them and compare it to the figures on the data plate of the air handler.