That’s true. But you probably can’t. At least any more than others. It’s a systemic issue in the ad network ecosystem which you don’t have much control over. If you can figure it out, odds are lots of others can too. People do assess the quality of traffic sources and do check the return on ad spend. It’s that system wide process that keeps the return on ad spend roughly constant.

The point here, for me, is that a microeconomic perspective on this whole question is more salient than a purely technical one.