What kind of evidence are you looking for here? The effect of financial incentives on human behavior is well-established.

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.

frankly, any evidence besides zero would be good, but ideally any evidence which is convincing. It's up to the conspiracy theorist, not the audience, to come up with such evidence, whatever it may be.

an example of evidence of such a conspiracy theory would be internal emails telling a reporter to go easy on the US government because a small portion of funding comes from it, or to attack X because the US government wants them to

statistical evidence would work too if it proves the claims of bias somehow, I leave it to you to figure out some examples

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

Ad Fontes tries to rate news media on the type of reporting it does and the general political skew. This doesn't capture everything, but it can be done. Except for specific topics I think it's slightly better than incentives in general, though incentives are important too.