Ask what kind of change in your bills you can expect to see if your HVAC is at the peak of tune.
Quote from a dead guy:
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."
If he says "It varies", ask what 10%, 50% and 90% of the customers have seen. If he gives you the "I don't know" answer, ask who does know.
ASHRAE is probably still credible for these kinds of estimates but their publications are costly.