I usually start by exporting to AME (as opposed to encoding "in-process" by clicking Export in the Export Settings dialog). Then watch the preview in AME's Encoding panel to see where it chokes. Adding an adjustment layer over the whole sequence with the Timecode effect applied--and scaled up to full frame width--can really help isolate the location.
That approach can be a pain in cases where it chugs away for a long time before encoding fails. In those cases, I use a divide-and-conquer approach: