Originally Posted by RChung

Yep, I feel your pain. This is a variant of, "If I can see it or measure it easily, it must be important; if I can't, it's safe to assume it's negligible." That's often a practical response; nonetheless, it took a long while for people to believe in the germ theory of disease cuz they couldn't see it, and that's probably pretty important.

There are some on-bike (nearly) real-time devices that can tell you what your aero drag is, but right now they're expensive and kinda finicky. That said, the issue is really the "real-time" requirement. If you don't need real-time and you understand how to analyze the data, you can do it after the ride. If you're careful, it's possible to estimate the differences between positions and equipment.