Lol. That's actually a very good question! ;)
I think maybe people just assume if they see a huge influx of views from Wanelo, for example, which you can tell by looking at your stats, and then also get a bunch of new sales around that same time, that Wanelo probably helped to get those sales there. It can't actually be proven though, since you're right that there's no way to trace where sales are coming from.
I think you can kind of tell if you add one advertising method at a time, and if you start getting an increase in sales, you can guess that that source is "helping" maybe?