How Do I Know Which Google Account is Used for the Market?
The Google Play Store allows use of multiple accounts.  In the app, press the "hamburger" menu button in the top-left corner (on the search bar) to select an account to use:



When you buy or install an app, you can expand the dialog to see the email address being used during the payment selection step (after being shown the required permissions).  You can still back out at that point.