One (indirect) approach to detecting / monitoring stray current activity is through the measurement of structure-to-soil potentials. The potentials are measured relative to a reference electrode in contact with the ground. A test lead has to be attached to the structure for such measurements, which is usually facilitated through test stations.

Dynamic stray current effects can usually be identified by fluctuations in the above potentials with time, and are hence relatively easy to detect. Typically, data has to be collected over extended time periods, at least initially, to assess the magnitude and frequency in potential fluctuations over time (for example stray current effects associated with urban transit systems tend to diminish at night when fewer trains are operational). An array of potential recorders (data loggers) can be strategically connected at test stations in a pipeline system, for characterizing stray current activity.

click on image to enlarge

The synchronization of such data logging devices, frequency of measurements, remote power sources (especially when measuring at low temperatures), securing such monitoring devices against theft, data transfer to a centralized computer and the fundamental suitability of the measurement instrumentation are all important elements of practical field monitoring. Solomon and Maddocks (see reference below) have presented fundamental data logger requirements for pipeline stray current monitoring.

Current mapping has also been identified as a useful monitoring technique in characterizing stray current effects and the source(s) of stray currents.    

References:

I. Solomon and S. Maddocks: "Data loggers and computerized systems for pipeline stray current monitoring", Materials Performance, January 1992, pp.23-27.

J.H. Fitzgerald III: "Stray Current Analysis", in Uhlig's Corrosion Handbook, Second Edition, R.W. Revie Editor, Wiley, 2000.

Links:
Stray Current Mapper, NYGAS Technology Brief at:
www.nygas.org/publications/99-716-1.htm