How do people track piracy anyways? I've heard it's pretty bad on iOS too (and a common reason to jailbreak) but it doesn't get much press since there are enough buyers.
Pretty simple. You look at how many people are hitting your analytics server vs the reported downloads by Google. That percentage is your piracy rate.
You can also trawl through the warez sites to see how many downloads there are for your software, this would give you a comparison of your pirate popularity vs others.
This method is simple, but incorrect. Google Play purchases are bound to account, not to device. If you purchase something and then install on your phone and tablet, you get two hits and single purchase, but both copies are legitimate.