In electrical engineering terms, we often define a component or material as linear if it behaves the same regardless of the magnitude of the excitation. For example, an ideal resistor and capacitor will act the same regardless of how much power I put through them. A diode will not, so it is a non-linear component.
You can do a quick sanity check of your data quickly based on this, if you're getting non-linear data but expect linear, you might want to check the specs of your system.