How are newtons measured?

Newton is a unit of force defined as 1kilogram-metre per second squared Mass is easily ascertained (weigh it), the other two variables can be calculated with a stop-watch and a tape measure, ie how far the object travelled in a known time. A 2 kilo object which travelled 10 metres in 2 seconds was therefore subject to a force of (2 x 10)/4 ie 5 Newtons