For Pluto, for what I know, they did use the technique of taking two images of the sky at different times and looked at them through a machine, but instead of relying on stereoscopic vision, they switched back and forth between the two, everything that isn't the regular star background would be moving or blinking.
So, relying on movement perception.
I remember reading that this was actually how Pluto was discovered: Someone compared a lot of photographic plates with this method.