How do I know if an account has transactions in a certain month?
Print the Account Balance report for that account (Reports > Accounts > Account Balance.
Select the correct date filter for you month, in some cases this will need to be Custom Date Range Specified.
If the report is blank then there are no transactions in that date range.
If you don't think the report worked correctly you can use a Custom Date Range that includes previous month(s) that did have transactions. Then you will see those transactions and if the last transaction is in a month before the one you were looking at you can be positive there are no transactions in that range of dates.