While doing your reports, a figure may appear that looks wrong, and you need to investigate.
This happened to me, a while ago while investigating our GST on purchases.
The way, I recommend you approach it is to isolate the problem to as small an amount and time as possible.
So what I did was break the period of the purchases into months and investigate each month, then in a month that was wrong investigated each week and then I went day by day till I found the error in the day.
Then I looked through all the transactions for that day till I found it, an insurance claim that had no GST.
This method always in my experience works.
Note the toughest errors to find in my experience are those that look right.