Though how do you try to validate which source is best to start with or is more reliable than the other....and when one problem arises, how or where do you look for a possible solution for let's say a particular problem in terms of historical accuracy or relevance? Coz I've read sources saying this and this but differs from the other. Like that time at Gaugamela. Some says that Alexander planned a feint to draw most or Darius III's men away in order to thin them out before turning back to pursue Darius. Some says it was an accident that became beneficial for Alexander...things like that.
Though I do realize that knowing what really happened word for word and action by action is almost impossible to account for, how do you then choose which sets of facts/theories/hypothesis to adhere to or believe was more plausible?
A full answer to this would fill a book, as it is the root question of historiography
Nor is there one way of approaching these questions, which is why there is rarely consensus among historians but I might offer a few hints.
How close in time an author is to the events he is relating is a factor, a contemporary may be under pressure to white-wash the powerful (a charge levelled at Onesikritos by the author of the Liber de Morte), but he may be an eye-witness to the events he is relating. Those writing after the death of a power figure, like Alexander, may not be under not be under threat from that figure but his influence may still persist, the rulers post-Alexander had all been his friends, except Kassandros. In general one would expect the accuracy to decrease over time, but this is contraverted if a late author goes back to early sources, as Arrian did.
This is more subjective but if an author is retailing extravagant tales and painting dramatic scenes, he is less likely to be as reliable as a more sober historian but this is not a hard and fast rule because...
Even sober historians have an axe to grind, and that can skew their view of things Polybios, for instance hated Kleomenes III of Sparta because he had defeated the Achaean League under his hero Aratos. Here one has to research the author and his times, most biases are quite apparent though, no one would be in any doubt about Xenophon's attitudes to either Thebes or Sparta! Ancient bias does tend to be as subtle as kick in the essential guts!
Related to bias and tone is the possibility of a secret agenda wherein the author grafts contemporary issues onto old stories; this is quite a common issue, how much is Tacitus portrait of Tiberius conditioned by his life under Domitian?
What opportunity did the author have to obtain accurate information? Note that the opportunity will not guarantee that it was taken! For instance, if someone is reporting a conversation that nobody else could have heard, it is not going to be accurate
This is even more subjective but some things are simply impossible; the numbers given for most barbarian armies are ludicrous, but they are ludicrous in every source!
Sometimes an ancient commentator will comment on the nature of a particular work, however, just like modern critics, the ancient ones have their own biases and this can lead to a slant that could be deceptive; for the most part the critics are concerned with 'style' rather than content, so Hieronymos of Kardia, an historian most moderns would give their eye-teeth to read complete (80 books, I think) was shunned for being dry and over long.
As you can see all quite vague, I could say that it gets easier with experience but the number of times that Xenophon and Paralus disagree with me puts the lie to that!
I concur on Manfredi - the purveyor of Alexander the Macho, and an example of the worst excesses of acritical use of sources, so many late fictions included, and poor tension - the Bosworth/Hammond rivalry is quite central to modern scholarship, which is best represented by Waldemar Heckel, he is fair in his representation of earlier work and is very good at citing his sources, though I would avoid the Osprey books as the format is too tight to allow proper discussion. Both 'Conquest and Empire' by Bosworth and Hammond's 'Alexander the Great: King, Commander, Statesman' are available cheap second hand on Amazon and e-bay.
When you think about, it free-choice is the only possible option.