How is the relative force of the fundamental forces measured? My physics textbook includes the following table:

My question is about the fourth row, where it compares the relative strengths of the fundamental interactions. How are these determined? Is the ratio of electromagnetic and gravitational simply the ratio of the force between 2 1kg point masses separated by 1m, and the force between 2 1C point charges separated by 1m? (that was the explanation my teacher  gave me) If so, how can this be justified, since the C and kg are just arbitrary units?
 A: Here is another table of fundamntal forces as used in particle physics.

Fundamental means the basic framework, and the basic framework is quantum mechanics, from which macroscopic forces emerge which can be demonstrated mathematically. These forces exist at the particle level, and their strength relative to each other enter in the Feynman diagram description of the complicated integrals that have to be computed in order to describe data at the particle level.
The column called "strength" gives the coupling constants that have to enter in a multiplicative  manner whenever a vertex in a feynman diagram involves the corresponding force. The coupling constants are measured from data.  Example here for weak  and here for electromagnetic. Mesurements are fitted by specific calculations, and thus the valuce of the coupling constants are defined. 
The relative strength is gotten from data that need two different couplings, so the Feynman diagram calculations  are fitted to the data, and the relative strength is determined.
In a hand waving way, the diagrams with the weak constant gives small crossections with respect to the ones with electromagnetic vertices, that is why the were called weak. The calculations quantify this.
Here is a link for measuring the strong force coupling.
Gravitation is still not definitively quantized, but the coupling constant is used in effective quantizations. Here is a link on how it is defined.
A: Of course anna v's answer is right, here are a few things I would like to add:


*

*EM force strength is measured from experimental data

*weak force strength is measured from experimental data

*gravitational force strength is not measured, but is only theoretically predicted

*strong force strength is theoretically measured and in experiments too, like with the exotic atoms, like the pionic atom. The pionic atom is an atom where around the proton, the electrons are replaced by pions. Since the pions are made of quarks and antiquarks, they show bosonic characteristics, and thus the nucleus and the pions are not held together by the EM force but by the strong force. This way they can measure the strength of the strong force too.
Please see here:
https://tel.archives-ouvertes.fr/tel-01674426/document
A: There is also another good answer re the strengths of the known forces in this blog
Summary:
Take two objects of some type, perhaps elementary particles, and place them a distance $r$ apart.  Suppose each exerts a force $F$ on the other.  Then we will say this force is weak if $F$ is much less than $ℏc\over r²$ where $\hbar$ is Planck’s reduced constant and $c$ is the speed of light.  
In short, for particle physicists:


*

*a weak force has $F r²$ much less than $ℏ c$   

*a strong force has $F r²$ about as big as $ℏ c$
