this poses an intersting, but not new, area of discussion.
several decades ago, I was faced with a similar problem on a large transport airplane - if you are looking for a ground loop, where do you attach the ground? In the case of this (350,000 pound class) airplane, I was "measuring" 200V spikes from ground to ground - looking at a shield grounded near the middle of the airplane compared to a "ground" near the nose (and all of this across what is basically a huge aluminum "ground plane"). that was a surprise to me - to this day I don't know if what I was measuring was real, but some tweaking of the shield made the problem I was really looking for go away.
What I would do is establish a "master ground" node somewhere and reference to that, and run a shielded wire (with the shield also grounded at the master ground) from the master ground to the scope - then you can attach the probe to any other "ground" you want and you will get an indication of what's going on - I say indication because you still don't know for real what might be induced into your wires even with shielding