So far I've never purchased anything from the Market but just installed free apps. Now, I'd like to purchase an app and I want to make sure the Market is using the correct Google account, since I have two (one personal @gmail.com and one from a Google Apps account for work). How can I do that? Do I just assume the first account in the list is the one used in the market? Keep in mind that I don't really remember which account I added first.
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.
It's probably worth noting that standard Google Apps accounts can't use Checkout at all, so you can't put a credit card on them, and so can't pay for Market Apps. Only Apps accounts upgraded over the last couple of months to the upgraded Apps accounts can (if your account was upgraded you should have got an email with the title "Your Google Apps account now works more like a full Google Account." If you haven't then the only account you can use to buy apps on the Market is your personal one.
If you visit play.google.com and click on "My Android Apps", you will be able to see which apps were downloaded for each of your accounts. On mine, my entire app list shows on my personal Google account, and only the system apps show on my Google Apps account. If you have a similar scenario, it's probably safe to conclude that your personal account will be used for on-device purchases.
You can buy apps on https://play.google.com/store using a browser. This way you can be sure with which account you are logged in.