Its isotopic composition would be measured with a mass spectrometer to determine its average relative atomic mass.
From the en.wikipedia.org
For example, the relative atomic mass of chlorine is 35.453 u, which differs greatly from a whole number due to being made of an average of 76% chlorine-35 and 24% chlorine-37.
From the en.wikipedia.org
Mendeleev arranged the elements in a table ordered by atomic weight, corresponding to relative molar mass as defined today.
From the en.wikipedia.org
Atomic weight expresses the average mass of an element's atoms relative to those of other elements.