What reports in BUSY can help me identify differences in filed GSTR v/s books?

For checking differences between Gstr data and BUSY you can check Reconcile Gstr Data reports in BUSY further find out the steps to check those reports in BUSY 
 Step 1: Open the company in BUSY then go the Administration menu and select Configuration. 
 Step 2: Choose Features and Options and then GST / VAT. Enter your GST portal username in the GST Portal User Name field and save it. 
 Step 3: Next, open the Transaction menu and select GST Misc Utilities. 
 Step 4: Choose Download GSTR JSON Data and select Download GSTR-2A or Gtsr2b or as per your requirement 
 Step 5: Click on the download button along with the month then you will receive an OTP on your registered mobile no and email id, enter that OTP and can download. 
 Step 6: Now, under the GST Misc. Utilities select the Reconcile GSTR Data option. 
 Step 7: Chose Reconcile GSTR-2A (Party Wise) / (Vch. wise) as per the requirements or any other report you wants to reconcile as per your requirement . 
 Step 8: Enter the period and can match your purchases with GSTR-2A. 
 Note: Please ensure the API access is enabled on GST portal to receive OTP.