Any ATCO worth their licence only needs accurate information from participating pilots to do that. And it's done well, generally.
How does the ATCO know that he is doing the job "well", if he doesn't have radar? All it takes is for some "participating pilot" to give him duff position info.
Of course, we know that pilots never give duff position info. Why would they? Navigation is always perfect, and nobody ever wants to get an earlier clearance to fly an approach (than somebody else they can hear on the radio who is also flying to the same place) because avgas costs virtually nothing
Many times I have
seen somebody give totally duff position info (within an ATZ) so they could beat somebody else to be cleared to the OHJ position, etc.
To IO540: many ATSUs will have a "discrete" squawk which can be allocated. This allows adjacent radar-equipped units to see instantly exactly who is talking to that a/c, and if necessary "request co-ordination."
Agreed. Listening squawks are a very good idea.