DRSBoard::GetTriggerSource() simply returns what has been enabled via DRSBoard::SetTriggerSource(). The actual source which causes the trigger is not stored in the hardware of the board, because I can be reconstructed easily from the waveforms. So just look which of the channels is above your trigger threshold. If none of the channels has a waveform obove the threshold, then the trigger must have been come from the external trigger.
I'd like to be able to know which channel (0,1,2,3 or external) was responsible for the trigger. DRSBoard::GetTriggerSource() seems to always return 1. Is there a way to get this information? Using the DRS4 evaluation board and software version 5.0.3.