Mechanical advantage is the ratio of force output from a machine divided by the force input into the machine. It therefore measures the machine’s force-magnifying effect. Actual mechanical advantage (AMA) can differ from the ideal, or theoretical, mechanical advantage when friction is taken into account. For example, the actual mechanical advantage from a lever will not be significantly less than the theoretical mechanical advantage, because there is no significant mechanism for losing energy through friction. On the other hand, a rope-pulley system may lose a lot of energy through friction in the pulley wheels.
Attach a Newton scale (it measures force instead of mass) to the input end of the machine in question. For example, for a pulley system, you’d tie one end of the scale to the pulling end of the tackle, or line.
Exert enough force through the scale to hold the load static and then take a reading. For example, you’d pull the tackle of a pulley system out a few inches just to lift the load completely off the ground, and then take a take a force reading off of the scale. This is your Force In.
Weigh the load directly with a Newton scale, attaching the load at one end and lifting the load by the hook on the other end of the scale. Take a reading when the load is static. This is your Force Out.
Divide Force Out by Force In. This is your actual mechanical advantage.
Some expositions on the subject refer to the Force Out as the “resistance force.” This is not to be misconstrued as a measurement of friction alone, but also including the load.