Constant concentrations in solution
A solution by definition is a homogeneous mixture, so its "concentration" is same throughout the bulk. Let us have a look as to how one would prepare a solution of a given concentration, say in 1 L. We would weigh out a known weight of a solute and dissolve it  ~ 500 mL of solvent, with mechanical stirring or shaking. We would add more solvent until the total volume is 1 L. We would stir it sufficiently, again, until we believe that the stirring has made the solution uniform.

a) If it a colorless solid, like NaCl, there is no visual way to tell if the mixing is complete and the solution is uniform. You have to stir it very well.

b) A colored solid, like copper sulfate or iron sulfate is more or less visible if it is not homogenous. Different colors in the bulk.

If you are still not (mentally) satisfied, you can take portions of the solution from surface, middle and bottom. Measure their refractive index and they should be identical. If they are not not, the mixture needs more stirring.

Edits:
Some people may wish to differentiate microscopic vs. macroscopic homogeneity regarding the definition of solutions. IUPAC does not offer such a distinction. For an analytical chemist or any researcher interested in measuring or making known concentrations of solutions- a solution has to have identical concentration throughout the bulk. Ultimately, it will boil down to semantics or the problem of "how many angels can land on the head of the pin" - a debate which has no end! en.wikipedia.org/wiki/…