my $ret = 1;

for my $d (1e-15, 1e-20) {
   printf("%g %d %d\n",
      $d,
      ( $d              )?1:0,
      ( $ret-$d != $ret )?1:0,
   );
}

##</code><code>##

1e-015 1 1   # Often equivalent.
1e-020 1 0   # But not when there's an underflow.