I'm taking what is usually called "freshman's" mechanics course, and I'm basically trying to understand how forces are measured and determined. Starting with the definition of the procedure for measuring a force (basically, the construction of a spring-based dynamometer), and after the statements of the Galilei's principles of relativity and the definition of the inertial reference frames, I find in my textbook the description of an "experiment" for determining the well known relation between force and acceleration, carried on a smooth inclined plane. Using a dynamometer, we can, by simple observations, conclude that the force measured in the tangent direction to the plane is proportional to a constant number $p$ multiplied by the $\sin$ of the angle $\alpha$. $$\lVert f \rVert = p \sin\alpha$$
The thing that I don't get is the following sentence:
"In such conditions (smooth inclined plane), it's simple to verify, using the dynamometer, that the resultant of the forces, $\mathbf f$, is tanget to the plane."
How does who wrote this concluded that the resultant is tangent to the plane, and so by putting the dynamometer exactly in such a position as in the figure, we are measuring the vector sum of all the forces acting on the object (the gray ball)?
[We can obviously say that there is the weight $\mathbf W$ acting on the ball, perpendicular to the floor, and that by taking the components of $\mathbf W$ the one that is normal to the plane is canceled by the reaction of the plane itself, making the tangent projection our resultant, but the book never saw a word about the reaction force of the plane (but there is instead a description of what is meant by equilibrium of forces: a point is in an state of equilibrium if there is no forces acting on him).]
p.s. Trying to generalize my question, i would ask (but this is less important): is there a way to determine the direction of a force by using a spring-scale/dynamometer?