Check Your Answer Against A Number You Already Know

The strongest verification available to a beginner is a figure that came from somewhere else entirely.

Before you go looking, find a number you already trust. Last month's total from a report somebody else produced. The count of active accounts that the operations team quotes from memory. A single invoice you can hold. Any figure from outside the work you are about to do. It does not need to be exact and it does not need to be recent. It needs only to be independent, because independence is the entire value: it was arrived at without your assumptions and cannot inherit your mistakes.

Then build toward it. Ask your question in a way that should reproduce the known figure, and see whether it does. If it matches, you have learned something real about your understanding of the data, far more than any amount of staring at your own working could tell you. If it does not match, you have learned something even more valuable, because now you have a specific discrepancy to chase rather than a vague unease. Chasing a discrepancy is tractable work. Wondering whether something is wrong is not.

Very often the mismatch turns out to be a definition rather than a fault, and that is a genuinely good outcome. Their number counted orders placed, yours counted orders paid. Their month ended on a Friday, yours on the last day. You have not found an error, you have found an unwritten rule that everybody was carrying in their head, and now it can be written down. Reconciling to a known number is the fastest way to learn what an organisation actually means by its own words.