How do you call "func()" vs "func() const"?

[ecspansion]

class Foo
{
  Foo() { }
  int func(int a)
  { return 0; }

  int func(int a) const
  { return 1; }
}

int main()
{
  Foo foo;

  foo.func();
}

In the example above, how do I know which func I'm calling? More to the point, how do I call the one with the const?