1.  Run the Aging By Customer report in summary:
-From the Report Manager select Accounts Receivable.
-Select Invoices/Statements/Aging. 
-Run the report in summary for the date you are trying to balance to.
-Run for all customers, uncheck “All” for date and run for the end of month date.


2.  Run the Aging By Receivable Account:
-From the Report Manager select Accounts Receivable.
-Select Invoices/Statements/Aging.  
-Run the report in summary for the same date, and for account mask with all x's.
-Run for all customers, uncheck “All” for date and run for the end of month date.
 
These 2 reports should balance.
 
3.  Run the Ledger report:
-From the report Manager select General Ledger.
-Select Transaction Reports.
-Run for the account numbers you have on the Aging By Receivable Account for the same date.
 
4.  If you do not balance, then run an Account Detail report on the GL accounts:
-From the report Manager select General Ledger.
-Select Transaction Reports.

You will most likely find there are entries made directly to the General Ledger Accounts Receivable accounts.  These entries should have gone through the AR module.

If the Accounts Receivable module is correct, then you will need to do journal entries to correct the accounts in General Ledger.
If the Accounts Receivable module is not correct, start by making entries in this module to correct any entries.  Once Accounts Receivable is correct, verify if General Ledger will need journal entries to balance.