If you log into your SWA account and click on the flight, you'll see 'change'. Click on that, select that flight and then you'll rebook the exact same flight. You'll see what the credit amount will be and you can abort the whole process anytime until you hit the 'are you sure you want to make these changes?' prompt.
BTW- your credit is tied to your confirmation number, you might need it when you want to use the credit. Keep track of it.