If one could hypothetically stretch / squash / in some way distort a piece of space, an outside observer could tell by looking at the distorted object/space that it had changed because the difference is evident relative to their own undistorted space. I.e., I can tell you have been distorted because I can from the outside compare you to my lack of distortion.
So, how would an object or observer inside the distorted bit of space be able to tell that they had been stretched or squashed? The space around them, and thus the coordinates relative to themselves, are distorted with them and are unchanged from their point of view (this is assuming they cannot view the outside, undistorted space). Right? E.g. if I could only see myself and my ruler and none of the rest of the universe, and both my ruler and I were scaled in exactly the same way, could I tell?
Secondarily: this stemmed from me trying to wrap my head around whether mass and energy distort only spacetime, therefore creating curved paths for objects in that space to follow, or if objects are also distorted. I always assumed the latter, thinking that spacetime and objects were not independent, but then started to doubt because explanations of GR make it seem like spacetime is bent and then objects fall into these bent paths, themselves unchanged (meaning that they are indeed independent). Then, I thought about spaghettification around black holes and that contradicts this!
Please help me organise this mess of thoughts! I love thinking about GR semantics and subtleties. I've just started learning about it and it is just beautiful :)