you could use a data logging wind speed gauge, ofcourse you also need to correct for variations in air density caused by fluctuations in barometric pressure, temperature, altitude, and humidity so you need to datalog those variables. As well as speed you also need relative wind direction because the drag factor varies with a crosswind, most of these variables can be datalogged with instrumentation used in yacht racing, any aerospace contractor would also be able to help you.

Ofcourse, carrying these instruments and their power source will add weight and aerodynamic drag so you'll have to develop a computer model that will correct for that before you can get your true performance. You'll need to establish some baseline performance measurements under controlled conditions to develop an accurate model, see if NASA can help, they have the wind tunnel and necessary instrumentation.

Let us know how you go.