Through observations and measurements, we identify physical properties at constant values without any deviation whatsoever.
The speed of light in a vacuum is an example. That is extremely important for observation. It enables us to see the world around us in a precise way, not just in our direct environment, but also over large distances. The velocity of light is such an absolute physical constant.
There is also scientific consensus that the gravitational constant G, the Planck constant h, the electric constant ε0, and the elementary charge e are absolute physical constants. We assume that these physical constants are invariable in space and time. For the speed of light, this has been proven over time going back to the Big Bang. For the other constants, there is a high level of confidence that we may conclude this invariability as well over extremely long periods of time, but those conclusions are based on secondary observations.
There is another group of physical constants for which deviations have not yet been found.
In particle physics, it is complicated to measure some of these physical constants, due to the small scale of these particles. The measuring method itself has a measuring tolerance that must be taken into account, but in general, there is consensus on most of the important physical constants as highlighted in the Standard Model.
There is no reasonable doubt among scientists about matters like the “invariant mass” of an electron and a proton.