How do I monitor a cron job?

Use a heartbeat (also called a dead-man's switch). Instead of something reaching out to your job, the job pings a unique URL each time it finishes successfully. If a ping does not arrive within the window you expect, the monitor marks it down and alerts you. That is the right tool for cron jobs, workers and backups, which have no public URL for an outside check to hit.

Set the expected interval to your schedule plus a little grace, so a job that runs a few minutes late does not page anyone. If your job can also report failure (a non-zero exit), have it ping a fail endpoint so a run that finished but failed is caught too, not just a run that never started.