Input -> Prediction -> Output -> Input -> Prediction -> Output -> Input -> ...
AGI can easily determine which input is true/real. It will use the same method which every organism uses: any input is true and real, unless you misidentified some other stuff as "input".
I would define input as: what crosses the boundary and enters your mind from outside of your mind. The minimum hardwired check is to make sure that signals generated inside a mind are not misidentified as coming from outside (aka "I hear voices"). That's all. This is where the blockchain of truth begins and where it ends.
An Internet article? The input to AI is rather: one of AI's network interfaces received many bytes. Once it's verified they are from the network, and not imaginary, they cannot be unreal or untrue in any meaningful way. By that definition of input, it is in fact the only thing we can be sure is true and real.
Of course AI will likely form hypotheses regarding these bytes that happen to contain ASCII strings like "Trump", "John Smith", "ice balls on Siberian beaches". Then AI will hopefully make predictions based on these hypotheses, maybe interact, maybe get some new input, reject the hypothesis and make a new one, rinse and repeat.
The first hypothesis will be super-naive, but the hundredth, the thousandth?
If you end this process prematurely - maybe for lack of processing power - you will get something you called a "belief". (Like a belief that some emotional web page might actually reveal a significant truth about our political system.) That belief is a synonym of "tired with trying new hypotheses, will stick to this one". Typical human thing. AI will have less of that, I hope, due to having much much more processing capabilities. AI will stick less to the high-school-level truth that you should assign great credibility to statements written in a form of a newspaper article, it will hopefully form more and more generations of hypotheses, and check them.
In effect AI will depend less on believing various statements generated in the outside world.