Thanks for getting back to us, @mechanicalflower.

Let's run the Transaction Detail Report to see the available credit of your vendors and customers. 

1. Go to Reports, select Custom Reports, then Transaction Detail.
2. The Modify Report: custom Transaction Detail Report opens automatically.
3. On the Display tab, add the Open Balance column, then remove other columns when necessary.
4. On the Filter tab:
5. Select the  Accounts Payable account.

The Open Balance column is where you can see the unapplied or remaining credit of your clients.

Once done, you can follow the steps outlined by my colleague @IntuitLily on how to link the check to the bill.

Also, I'm adding this article on how to create, modify, and print checks. It provides steps and figures that will guide through the steps.

Keep in touch with me here should you need any additional assistance, I'm always around to lend a hand. Have a good one.