You’ll find the logs in <i>QPJOBLOG</i>. To view the job logs — type in <i>WRKOUTQ QPJOBLOG</i> and then press enter.
Previous suggestions might give some general clues. If you need to be certain, there are options that must enabled on the system.
First would be the system audit journal — QAUDJRN. System value QAUDLVL should specify *PRTDTA (and possibly *SPLFDTA) to log audit events related to printing.
Better and more direct info could be retained in the system accounting journal — QACGJRN. Tracking printer usage is enabled with system value QACGLVL which should include the value *PRINT.
With the accounting entries, you can get very detailed info about who was printing how many pages of what reports on which printers during dates/times of your choosing.