You may know by a penny how a shilling spends.

The saying proposes that a small example of spending can reveal the pattern of a larger sum. Observing how someone uses a penny is presented as a clue to how they will use a shilling.

It encourages drawing a practical judgment from a modest test or sample. A minor expense may disclose habits that would become more noticeable when greater resources are involved.

Use it when a small purchase, trial, or early action seems enough to indicate a person's likely approach on a larger scale. It can caution against judging only the amount involved rather than the pattern it reveals.