Some multi-story buildings in my region require earthquake isolation on some of the piping. Thus my question. Is there a way to determine whether a nearby metal water pipe is actually Earth grounded, versus floating? Does this make a difference in terms of finding a suitable ground for a non-ground-floor HF radio and antenna?
"Electric" ground and "RF" ground are different creatures. Water pipes may make a good RF ground if one is close to the point where the pipe goes underground. A floor above and it will not be (as) effective. Dropping a grounding strap/braid out of a convenient window to reach a real grounding rod is recommended. That being said, connect the ground and try it - sometimes you have to work with what is available or possible. If it improves performance, use it - or run without a ground if it works (sounds like an indoor or balcony antenna, so lightning hits are not a concern).