A service I recently discovered is URL Canary. It creates a honeypot URL that you can then put in a location such as your cloud storage. It alerts you if that URL has been accessed.
URL Canary will catch automated robots and crawlers, as well as manual human attackers. The only time it won’t catch an attacker is if they don’t see the canary, or they don’t find it sufficiently-compelling and opt not to visit it. Since you have control of the URL and the domain name, you can make your canaries as compelling as possible for your specific use case.
There’s a similar service I know of called CanaryTokens.