A resistive loading will attenuate all the signals equally, independent of their frequency. Only a frequency selective network can reduce one signal more than the other if their respective frequencies are different. What ever the amplifier loading, the signal source will be looking into a resistive load and the induced signal will see a L+Ri load, the L being an unknown parameter, totally dependent on the actual field conditions, so in effect unpredictable attenuation. As in case of your query, a notch/band stop filter, that is one which kills 50Hz, is one part of the solution. The other is shielded cable, ferrite beads, common mode chokes, differential amp(read EM's post) etc. In fact if your signal does have a 50Hz component,then there is no DC way to distinguish between the signal and induced pickup of mains. It would be far better to encode it (analog conversion or digital coding) locally, then send it out to the control system.
An extreme example may make it more clear, the principle of modulation and demodulation and the public radio/television system based on it. Here the recieved signals are in mV and uV, yet the music can be heard with crystal clear clarity except in extreme conditions like thunderstorm.
If the equipment have no marking of standards, it better to be safe than sorry, assume no classification unless tested otherwise.