According to infoplease.com, elements can be investigated using a range of different techniques to determine their fundamental properties. I'm assuming this list isn't exhaustive.
Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number.
Are there any two stable(ish) elements that are indistinguishable in terms of their fundamental properties by a single measure? That if I investigated them to the limit of human technology in that one specific measure, that I wouldn't be able to tell one element from another?
For example, Neptunium and Plutonium have very similar melting points. I wouldn't be able to tell them apart unless I have a thermometer capable of measuring temperatures to an accuracy of .5 degrees celcius.