I cannot go into the details of evidence gathering as the methods we used are still relevant today. What I can say is that we did not stand in railway sidings all night on the odd chance that siding could be the "target for tonight".
I'm pretty sure I know how and what methods were/are used. Today I would say its even easier than before, thanks to the likes of .... (If its what I'm thinking of) BTW One method was widely televised.