The joblog for the job would have an entry stating something like ‘JOB nnnnnn/userid/name changed by user USERID’. It doesn’t specify what was changed, but you can see who is changing the job in general. This will be available while the job is active. If the job has completed, the log setting would have to have been configured to save the joblogs. i.e., LOG(4 0 *SECLVL). Quite often this is set something like LOG(4 0 *NOLIST) which means no job log is saved if the job completes normally. If this is the case, change the job description used or if necessary the SBMJOB command to LOG(4 0 *SECLVL) and the job logs will always be saved.
Our system adminstrator is tightening down on security and rarely allows *JOBCTL authority. Without *JOBCTL authority, you can’t change, end, or look at anyone else’s jobs. You might see who’s got *JOBCTL authority that doesn’t need it.