For the last two trips I've booked, I've gotten the booking codes/reference numbers for each GDS used on the booking out of the paperwork I got from the travel agent.

I've had this info appear on the Electronic Ticket Receipt for the last 4 trips I've taken which used different GDS on the booking with each trip issued by a different source.

The info you're after should be listed next to each flight on the Electronic Ticket Receipt. If you didn't get a receipt or it doesn't contain this information, then you'll have to contact the airline or agent you made the booking with.