This kind of what I am thinking too. It's not the easiest thing to do but you need to try and look down the fuselage towards the tail and see if there is any evidence of repairs to the plane. You can get an inspection mirror from just about any auto parts store that would make it easier to look down there and see if there is any damage. Also look at the covering on the plane and see if there is any evidence that it's been recovered. On ARF's there should be no seems in the covering along the length of the fuselage. If you can see where covering has been added in along the fuselage that would be a dead giveaway. Also look to see if the color of the covering on the rest of the plane matches the covering on the tail of the plane, if not it's another clue that it's been recovered.

One thing to try would be to have a helper hold the plane up with the engine running and open it up to full throttle. If the tail surfaces shake as you described then you've got issues with bad structure or vibrations from the motor. But if they don't shake then you have issues with flutter occurring while the plane is in the air. Flutter can and will destroy a plane if it's not corrected. A few things to check that could be the cause of flutter are failed hinges, loose or weak connections on the control horn, flexing of your pushrod, loose or weak connections at the servo control arm, failing servo, or stripped or loose control arm on the servo. No matter what, a plane with a known flutter issue shouldn't be flown until the cause of the flutter is determined.

Hope this helps

Ken