I think pretty much everything has been covered on this thread, but I thought another anecdote might provide a different view of the same concept.
I just re-qualified on the MD-80, thus gaining an opportunity to refresh my original instrument scan from years gone by, and relieving me of the need to remember what all those colored little bugs and notches and hash marks are supposed to mean.
However, the check airman demonstrated a simple technique for checking your actual weight before beginning the approach. The 80, of course, has an alpha protection mode incorporated in the autothrottle speed mode, so that it will theoretically prevent you from decelerating into the protected stall margin by adding whatever power it might be able to scrounge up. We manually bug up the min maneuvering speeds prior to approach based on what we think we weigh, which is of course paperwork plus known fuel. If, upon dialing the speed back for the first speed reduction, you get an ALPHA annunciation on the FMA at a speed somewhat higher than the min maneuvering bug, then you know you weigh more than you think you do. The computer is telling you that you are already at a higher angle of attack than you think you are, and thus the protected stall margin is displaced to a higher speed range.
What you hope to see is the ALPHA annunciation illuminate just as the speed bug is dialed down past the manually set min maneuver bug; that says your weight is correct.
John:
Just depends on which way you apply parallax to your contemplation.
What a marvelous metaphor...must keep that one squirreled away for future use, with appropriate acknowledgement, of course.