I've always maitained that the first aerodynamic sign of the approach to a stall is an increase in back pressure on the controls.
If you haven't changed the centre of gravity or configuration and the aircraft was trimmed to a certain speed then any increase in back force must be because you are slowing down (increasing pitch attitude).
The next symptom is a reduced effectiveness of the controls.
Then you usually get some kind of airframe buffeting.
How many stall warnings do you need?