Noise - this is what interferes with the signal.

To say there is noise or not, you need to eliminate the signal. All that will be at the output of the sensor in the absence of a signal is noise.

Best of all, noise can be characterized by its spectral and temporal characteristics.

Two common standard methods:

1. Spectral density analysis (usually log scale used), allows to see e.g. 50Hz, 100Hz etc lines

2. Amplitude analysis (histogramm of in put values) - allows to see ADC problems, repetitive pulsed interference