Succinctly, you don't or can't tell from inside the function which variables were used to create the argument to the function. There's no guarantee that the argument is a variable, or a single variable — for example:
foo 123
foo ${HOME}:${PATH}
Further, if the function cares, it is probably mis-written. It should be as nearly self-contained as possible, and therefore independent of such issues. If it isn't, it suggests that maybe the function is not functionally cohesive.