You can bounce radio waves of objects in our solar system to get very accurate measurements, parallax measurements will get you up to about 100 light years, then you're into main sequence fitting which relates the apparent brightness of known types of star to the distance. But to get the really big distances, you need to use Cepheid variable stars which pulsate with a rate very accurately associated to their brightness and by measuring their apparent brightness you can get distance. You can also use supernovas since their brightness is calculable. But you're right, we don't have a ruler that big and a lot of this is open to error, possibly a very big error.