The time to check if the fuel caps are in place is during the "golden lap" just before the PIC, as the last person aboard. That is when latches, doors and fuel caps are checked secured.
I would have thought just before t/o would be leaving it a little too late, if remembered at all. However during a control check of the ailerons (full up) would be a time for a final check. Difficult in a high wing I would expect.