Meanwhile case-specific evidence is largely unavailable. Reporters would be loathe to admit to being cowed, but are also aware that corporate executives make decisions on things like promotions and timeslots based on the bottom line, or in less well-funded entities that a loss of funding can directly lead to a loss of employment.

How about statistical evidence? Well, if Republicans are threatening NPR with the loss of funding, they could slant their coverage to placate the legislators criticizing them, or they could slant it the other way to bolster their support from the other party. In either case it compromises their neutrality, but now you can find "evidence" of this in any divergence from neutrality in either direction. Since they couldn't reasonably be expected to be infallible in the alternative, that doesn't prove anything.

So you make people aware of the incentive, because that's all you're really going to know about in practice.