Interesting to note from the NASA video that signs of a tail plane stall are reduced when using powered flying controls like the Q400's elevator system and that the possible only sign will be an unusual trim setting which could easily be missed as has already been pointed out previously.
Not sure if it's been answered in a previous post, but:
The Q400's autopilot will trim the forces in pitch. The yaw damper will to a limited extent help reduce yaw. The roll is held by the roll control actuators, excessive trimming of forces is indicated on the PFD, but no trimming is done by the autopilot. A manual pitch trim will disconnect the autopilot. A manual roll trim will not disconnect the autopilot, although the correct method is to manually disconnect the autopilot and trim out stick forces. However it is possible to trim in roll until the message goes away