Below link might help you in identifying the Job log for Prestart Jobs.
Finding job logs from TCP/IP server prestart jobs
You get joblogs for prestart jobs the same way as for any job. If the job is active, you can run DSPJOBLOG OUTPUT(*PRINT). If you want joblogs for new prestarts, set the job description LOG() parameter to spool new joblogs. If joblogs were previously spooled, use WRKJOB to list the jobs of the prestart name or use WRKOUTQ to go into your joblog *OUTQ.