Intuitively, I'd say you can't. What about a sub like this:

sub bizarre {
  if ($_[0] eq "context-sensitive") {
    return wantarray?["sna","fu"]:"snafu";
  } else {
    return "bar";
  }
}
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which is sometimes context-sensitive? I suppose you could mark a sub context-sensitive if it is for at least one set of parameters, but that might have limited value.