ORIGINAL: pimmnz
By measuring the drag
Seriously, the only practical way of doing this is to find a university handy that has an aeronautical thread and a tunnel big enough to put the whole model in. Knowing the tunnel corrections they can the measure all the parameters at various airspeeds and come up with Cd and Cl for the model. They can do lots of other things, stall speed and stall aoa etc. Could be a useful trade off here, it gives the students a practical set of problems, and you all you need to know about your model, and the cost of doing this, which could otherwise be horrendous, may be within some sort of bargaining reach. It may mean you lose the model for a month or two, but the results could be well worth the time. Based on what you find, the answers could be extrapolated across a wide range of models, and answer quite a few questions.
Evan, WB #12.
Pimmnz I really did think about this option for oh, about a quarter of a second. Then my no voice took over and said it's way too expensive, the prof's will laugh at your toy airplane, and so on. But since you mentioned it I decided I'd do a quick search "UT windtunnel" just to humor you. Over the last 3 hours I've learned there are THREE low speed windtunnels within 10 miles of my house! I also learned there is a 3rd year undergrad course taught using the downtown facility 6 blocks away that sounds like what you were suggesting. The students are required to mount, test, and report on airfoils and bluff bodies as a key part of the curriculum. It's the weekend but I plan to contact the course instructor and see if I can get some of my stuff on the list. One other thing I was amazed to see is the "real world" emphasis on all the coursework that has taken place just in the last couple of years. The aero department has for the last couple of years been sending teams to the Design/Build/Fly competition and have even been using my own club's R/C field to test prototypes.
Thanks for the knock to my noggin!