How do we know that the Voyager spacecraft have reached the heliopause?

They haven't quite gotten there yet, but the Heliopause 'bow shock' where the solar wind slams up against the interstellar medium, produces a variety of plasma waves, and these accelerate charged particles in the wind. It is the distant traces of this interaction that the spacecraft see in the local flows of particles and fields in their immediate vicinity even though they are still millions of miles from the bow shock region itself.