How do you know if a stamp is cancelled?

A stamp is considered cancelled if it has a mark or imprint that indicates it has been used and cannot be reused for postage. This can include postmarks, ink stamps, or other types of markings that typically include the date and location of mailing. Additionally, a cancelled stamp usually shows signs of being affixed to an envelope or package. If the stamp is intact and shows no signs of cancellation, it is likely still valid for postage.