What do flight attendants do if a passenger asks what is going on in case of a potential emergency?
What's the general protocol for cabin crew, when asked by passengers
  who notice potential issues like this and get curious?


The flight crew and pilots are under no obligation to inform the passengers about non critical issues and broadly speaking it's often better to provide only needed information so as not cause some kind of panic. In the vast majority of situations on an aircraft there is nothing the passenger can do other than evacuate when told to do so.  

Turning an aircraft around and heading back to the point of origin is not necessarily an emergency although it can be there is nothing to say it is or is not from a passenger point of view. 

Even if a passenger insists the cabin crew will likely ask them to take their seats, buckle up and await any further instructions. It's possible that the cabin crew is not even informed of anything as the pilots try and work the problem. 



The later part of your question is answered here and the general answer is, unless you are a trained pilot it's hard to actually know if something is going wrong. There is a very interesting example of this from the Air Florida Flight 90 incident which is the only incident I know of where a non flying pilot spotted an issue and took action, this would be a fairly rare case; 


  Survivors of the crash indicated the trip over the runway was extremely rough, with
  survivor Joe Stiley – a businessman and private pilot – saying that he
  believed that they would not get airborne and would "fall off the end
  of the runway". When the plane became airborne, Stiley told his
  co-worker (and survivor) Nikki Felch to assume the crash position,
  with some nearby passengers following their example.