Just do the last 3 months (app. 40 receipts), or just the last month while you set things up and then add months as necessary. If a pattern unfolds you're done, if not delve further. Usually the culprit is pretty obvious with minimal work.
I visually scan my receipts at the store, just to look for price mistakes. Store policy is $10 off or free if I find a mistake. It's made me pretty good at remembering prices.