You can check the remaining credit balance on your account using the My Digicel app as follows:
View your balances by clicking here.
Step 1: Click on “Connect with your Digicel ID”
At the mid section of the page there is the tab "Available Credit", from there you can view your balance. There you will see the expiration date of your credit/sim card as well.
In case you have an active data plan, you will also see your data balance. The same for your bonus credit, that is displayed as "Free Credit".