So, in an effort to detect when the Voyager probes might exit the heliosphere, scientists have kept a watchful eye on two key pieces of data - particle energy counts and magnetic field strength (and orientation).
In 2004, scientists realized that Voyager 1 had traveled through the "termination shock" - a region where the solar wind begins to interact with the interstellar medium. Then, in 2010, the probe crossed into a stagnation region just beyond the termination shock known as the "heliosheath" - this is where the solar wind slows to zero and the magnetic field becomes compressed and begins to fluctuate.
According to theory, as the magnetic field begins to fluctuate, the number of high-energy cosmic rays should decrease inside the heliosheath - charged cosmic rays entering the solar system should become scattered by the magnetic fluctuations, decreasing the number of detections by Voyager 1. Looking at data through 2010, the researchers actually found the opposite to be true - as the magnetic field became more chaotic, the number of high-energy particles increased.