Come up with some new product that requires some personal data for usage (eg. age, gender/sex, address). Start to advertise this in just one country to one demographics, and look how many out-of-target orders you get.

Maybe it's even enough if you simply just sell it via mail order, you can then look at the addresses.

There's probably a natural information spread in any market (word of mouth, trade magazines), and there's probably a physical dispersion of the target group of people too (people move, visitors/tourists saw the ad/product and order it at home), but it still should be a valuable to see how much effect just one campaign has.

Maybe one of the best products for this could be a car. They are pretty standard, really not much difference between them, they are in all price ranges, and regularly new models come out. Advertise one in a few major US cities but don't in others.

For most vendors, not possible to accurately target age/sex/address online. They will certainly sell you the option, but they won't tell you how accurate the data is - or they will provide it a high accuracy with extremely low coverage.

To see this in action - spend a day watching pitbull videos on youtube and see how many spanish language ads you'll get.

Hm. How about simply buying ad space at a few local newspaper sites? Sure, it's noisy as hell, but maybe still a decent proxy variable.

Plus, maybe it's possible to somehow offer a coupon for those who buy via the ad, and so on.

After all the ad/tracking industry probably have a thousand tricks to increase accuracy of this.

But, yeah, I'm not holding my breath for a conclusive answer on this.

Traditional brand advertising testing (TV, newspaper, etc.) would be geography segmented as I understand it. So half the cities got the campaign and half didn't. You can mimic that with IP based geo-location although you'd get more leakage than pre-internet.