I guess that's not easy. In my case I experienced the same issue. According to some information I read, it looks there are several procedures Google uses to locate the user. One of them consist of to check the procedence of the Credit Card registered in Google Play, if users has any one. So if that is your case, you will be always shown with the prices of the locale corresponding to your credit card.
Second check, is made thru Sim card of the mobile phone. If you don't have Sim card or device is a tablet without Sim card, then the next step is looking to the WiFi connection.
Localization of the WiFi spot, and IPAddress, is used also to geolocate the user.
Finally if nothing of this works, then locale settings in the device is used. So in this way, Google show the prices according to the place where you are, which couldn't match with your locale settings in your device.
For example, if you're spanish, with your tablet with local settings in Spanish, and you're right now in USA, it has sense to show you the prices in Euros, because that will be the price charged in your credit card, and you will understand better the currency conversion, because you use euro in your life.
I hope this helps you.
So as a resume, how to test it? Very difficult.
Best regards.