Generally the first question is whether the sensor ground is the same as the measurement circuit ground. If you measure no continuity (a few ohms or less) between the two grounds with an ohmmeter, then they are isolated. The next question is whether the two grounds (commons) can be tied together. That can generally only be determined by knowing the circuit connections to the system for both the sensor and the measurement circuit. Often, to eliminate ground loop noise, a differential input is used by the measurement circuit so the sensor common can remain floating from the measurement circuit common.