I recently watched a video detailing some near-death encounters Alexander experienced along his campaign that seemed like the sort of material that would be found in a novel rather than a history book. So, do we know where the line can be drawn between propaganda, myth (exaggerated and amplified by later rulers of Ptolemaic Egypt, the Seleucid Empire, etc.), and legitimate historical events that actually occured during Alexander's campaigns?
There's one major source for Alexander's life, and that is Arrian's Anabasis of Alexander. It is, according to Arrian, chiefly based on Ptolemy's account, which appears to have been the best source that existed in antiquity. Other writers, such as Diodoros and Plutarch, drew more on the "epic" accounts of Cleitarchos, which is reflected in their writings. Therefore they don't necessarily provide much of a useful comparison.
While Arrian is generally assumed to have been faithful to the sources he used, actually evaluating the material is incredibly difficult, simply because of the lack of anything to compare to. There are major shortcomings in the account, certainly - Arrian glosses over central issues like why the hell Sardis, the single most important fortification in Asia Minor, just shrugged and surrendered to Alexander before he had even reached the city. Virtually no insight is given into the internal tensions of the Achaemenid Empire which must surely have plagued Dareios' reign, given the turbulence that preceded it and the irregularity of his ascension, and there's little to no useful basis on which to evaluate what military strength he was actually able to muster.
Therefore we are inevitably left with a Hellenocentric, triumphalist account that is very difficult to evaluate critically. So while there's a good deal of material that we can probably take to reflect historical events, contextualizing and critically evaluating that material is a very challenging, or even impossible task. The historical tendency to take Alexander's purported genius, "great man" status, benevolence and so forth as granted does not exactly help.