Even the question is hard to define for those who aren’t working in the field. What does it mean to be in the solar system? What do we mean when we say that it is not? The answer is: the solar system is defined by the region of space that is primarily influenced by our sun, Sol.
To leave the solar system then, means that Voyager has left this sol-influenced space and is now cruising through interstellar space. Practically, this comes down to the density of particles being much less in interstellar space than it is in the solar system. This difference was recently marked by listening to the tone of a stellar burst traveling past the probe. Determining this has taken a lot of data analysis from secondary sensors that were not initially defined to interrogate this question. Unfortunately, the equipment that was supposed to do this broke down some time ago. Rather than focus on the problem, what is truly remarkable is that this vehicle has been traveling approximately 38,000 mph for the past 36 years and it is still transmitting signals back home (from roughly 17 light hours away or over 11 billion miles).
Dr. Ed Stone describes the probe itself, its mission and the data behind this most recent claim that it has exited the solar system in this video: