depending on if there is a home-grown program (probably CLP) running the back-ups, you can look for the last time that job was run, how it was submitted, etc.
the job could be in you Job Schedule Entries (WRKJOBSCDE) and this will tell you when that particular job (or jobs) are scheduled to run, out of what subsystem and so on.
if you have ROBOT or other 3rd party software, you will have to look at their version of job scheduling.
You could also check the GO BACKUP menu. Enter option 10. Set up backup, then 20. Change backup schedule. Check if the Run backup using this schedule is set to “N” or “Y”.