Western Allied and German airforces kept very detailed combat logs. Claims of victories had to be documented, and of course combat losses were noted as well.
So when a DO 217E, Number 5354 U5+MS, commanded by Oblt Lämmel, was lost on 01-17-1942 after 18:35 hours (time of last radio call) somewhere near Great Yarmouth, you would compare that to the British combat records. If you then find a claim by P/O Norsworthy from 85.Sqd of one killed German bomber for the same time & area and no other bombers were lost nearby at the same time, the probabilty is very high you know what happened.
Of course, it's not always that easy.