If you don't have a way to measure wind speed and direction on the bike it will be hard to figure out the effects. Having said that, in one of my early attempts I used a triangular "loop" course, and I'd done enough tests on nearly calm days that I figured I could do "virtual wind" rather than "virtual elevation" estimates on non-calm days as long as the wind was constant in speed and direction. It turns out that even small swirls and gusts make the analysis hard enough to be kind of discouraging. Maybe I could do it on a slightly windy day if I used a big open flat parking lot without a lot of wind-intervening obstacles -- but I decided to wait until I can measure wind speed and direction. So now I try to test only on calm or nearly calm days.

[Edit:] Let me clarify: when I first started doing field testing, my position was so horrendous that there was lots of room for improvement and I could get by with pretty lousy technique and not entirely calm days because the changes I was making were pretty big. After the low hanging fruit got picked I needed to be more careful about all the things that can add noise to the drag estimates. Basically, the smaller the change you're looking at, the more careful you have to be about testing. So, if you're just starting out, yeah, you might be able to learn something from testing on not-completely-calm days. However, after a while you're going to have to be more careful.