How do you check the Newton pressure on a fire door to see if it meets the 90 N limit. This is in regards to the fire doors of Ontario Fire Code.?

Er... you press on it and see. If you can press on the release bar with 90 N of force (basically 9 kg or 20 lb) for 15 seconds without the door opening, it fails. This includes the pressure of the door latch release and any spring/hydraulic door closer. (For the latter, only force to initial travel is an issue, not force to open wide.)

You can often get a statement from a manufacturer, but you can also measure directly: just get a cheap postal scale with enough capacity, turn it on its side, and press with it. Multiply the reading by 9.8 N/kg. That won't be super-accurate, but as long as the measured force is below 90 N with more safety margin than the inaccuracy of your measurement, you've proved it. (Don't forget that the scale will read low, i.e. negative, when turned on its side, so you have to re-zero it or mathematically adjust for the negative starting point of the measurement.)

As a practical matter, most fire door hardware is far enough below the limit that a very crude measurement is sufficient; the fire inspector just presses on it with his or her "calibrated arm" and can tell that there's no need to measure more precisely.

If you're working close to the limit, the NRC probably knows about test procedures.