It seems that many manufacturers are moving from CFM/MPH claims to using newtons of force for comparisons. Does anyone know how they're measuring it - what tool/meter, from the motor or tube, etc.?
A blower, in engineering land, is generally characterized by two specifications, the flow rate and the pressure. The common methods of communicating them is either by listing the maximum values or through a blower curve. It sounds like these manufacturers are only giving you part of the story.
In practical terms, the pressure (which as blacksmith37 notes should be in N/m^2 or psi, not N) represents how hard the blower hits the leaves. Using a pretty simple equation (h=p/r*g), you can convert this to the more intuitive "if I use this blower to shoot a leaf straight up in the air, how high will it go?" More pressure means the leaf goes higher in the air.
The flow rate on the other hand tells you how much air is blowing, or practically, how many leaves you can blow at a time.
So if you had a choice of blowers, you would want a high-pressure, low-flow one to blow a small amount of wet leaves, but you would want a low-pressure, high-flow one to blow a huge amount of fluffy leaves.
As a bonus, multiplying the pressure by the flow rate (and throwing in an efficiency factor) gives you the power consumed by the blower.
Specifically answering your question, an engineer would use a differential pressure gauge to measure the pressure generated by the blower. If you were a sales engineer instead of a real engineer, you could then multiple that pressure in N/m^2 by the cross-sectional area of the blower outlet in m^2 to get a total applied force in N. That would be dumb though, because the pressure is a much more useful specification than the force.
A Newton is a force ( 0.22 pounds or 102 g, per Wikipedia). But you need to have a standard area ; as in water pressure given in pounds PER SQUARE INCH. I would guess since they are using metric the area unit is a square meter . In that case a rating of one newton would be a pressure of 102 g on an area of one square meter . That seems a reasonable order of magnitude for an inexpensive blower. They measure the pressure in the discharge nozzle.