Even humans use few relatively simple heuristics to decide what to trust.

One is that objective truth is internally self-consistent. If one AGW denier claims it's the sun, and another AGW denier claims the NASA falsifies the data, and they support each other, then you can judge these are conflicting claims and decrease your trust.

Also, false claims usually focus on attacking competing claims than to come up with a coherent alternative. And they tend to be more vague in specifics (to avoid inconsistency), compare for example vague claims about all scientific institutions faking data vs Exxon files containing detailed reports for executives.