How do I choose whether my payment form will run in live or test mode?
It's a URL difference, yes.

From Contributions menu > Manage Contribution Pages, you'll see a list of each contribution page, with a number of menus ("Configure", "Contribution", "Links", "More") next to each.  Select Links > Live Page for the live URL, Links > Test-Drive for the test page.  The URLs look like:

Live:  https://example.org/civicrm/contribute/transact?reset=1&amp;id=1

Test: https://example.org/civicrm/contribute/transact?reset=1&amp;id=1&amp;action=preview