There are a lot of factors that determine reliability of sources. I can’t speak for more modern historical sources but for ancient sources some considerations (though by no means an exhaustive list) will be:

Provenance: where did the source come from? Who is the writer and what is their background? Or if it an archaeological source where did it originate from?

Intent: is the writer attempting to write a factual, objective historical account or are there other intentions we need to consider? (i.e to write for entertainment).

Chronology: are they a contemporary source? Did they live through the events they are discussing or are they writing at a later date and using earlier sources of information?

Access: even if the writer is a contemporary source, would they be able to access the information they are discussing? If they are not a contemporary source then what information are they using to write their account?

It is then a case of looking at the source itself with these intentions in mind, and considering whether they hold up to our historical standards.

Let’s look at a case study, the famous historian and (arguably) the father of the scientific process of historical inquiry: Thucydides.

Provenance: Thucydides was an Athenian, born some time in the mid 5th century BC, he served as a general for Athens during the Peloponnesian War against Sparta in the latter 5th century.

Intent: Thucydides claims he intends to record the causes and events of the Peloponnesian War, with a focus on objective historical facts rather than the more Homeric intent of glorifying the past which was an influence on previous writers such as Herodotus. One thing that Thucydides does well which lends to his credibility is he explains his intention at the beginning of his work. He explains for example that because he will not romanticise the past like some of his contemporaries his work may consider less appealing to his contemporary audience, but for him he is content if future generations consider his work useful for gaining an exact knowledge of events. He also explains that whilst some of his contemporaries rely too much on eyewitness testimony to write their work he will try to only focus on events with which either he himself was present or use more credible informants with which he can corroborate information. This all lends to his credibility as an historian. However in other places we must be more careful on relying on his work. For example he mentions that when he uses set speeches in his work he and his informants had problems in reliably remembering exactly what was said and so he will relate to us speeches with a “habit to make the speakers say what was in my opinion demanded of them by the various occasions, of course adhering as closely as possible to the general sense of what they really said.” i.e he will try to stick close but it may not be exact. This then has led modern scholars to question how reliable the speeches are. For example the famous Funeral Oration of Pericles related to us by Thucydides in Book 2 has been debated as to whether this was the genuine speech given by Pericles or a rough iteration or what he said embellished by Thucydides own rhetoric.

Chronology: he is certainly contemporary, which is perfect, but around 424 BC he was exiled by the Athenians for his failure to defend Amphipolis from capture by the Spartan general Brasidas. And so this will affect his access to information.

Access: Thucydides was not only a contemporary but a general in the first phases of the war. In some cases therefore he would have first hand information of what happened, which of course is perfect. However as I mentioned because he was exiled early into the war his access would presumably be limited as the war progressed. However this did give him the chance access a wider range of people then had he continued serving, for example he would be able to speak to individuals on the Peloponnesian side of the war as well and provide a more comprehensive account. Largely though his information is from an Athenian perspective, and this shows in places with the amount of evidence he is able to provide for the Athenians and their actions vs the Spartans. He even mentions the secrecy of the Spartan state in determining facts such as the number of their forces. This does affect our perception of the war as it appears that the Spartans are more reactionary in their military tactics than the Athenians, but this may be because of the way he presents his information and his access to Peloponnesian sources.

Hope this helps in any way!

First: all source are reliable, even if it is a fake source like the protocols of elders of zion. More than the veracity of the source, we can understand some mentality of a certain period of time, for this kind of understand, even the notorious fake protocols could be a reliable source, because it all depends of your reasearch goals and questions.

Second: the commitment historians have are about understand and explain the past, not a commom search for the true (despite their existence). We are not lawyers, we are researchers.

We need to aproach the source always from a critical sense, there is not a more truly source then another, but there are sources that answer better some questions then another. For exemple, I work with oral history when I need to figure out more particular points like the feelings about some fact, because that's the only way. But, the interview is taken not around the period I research, but some years after, so the narrative the person give to me are much more their memory and vision from the past than the past it self. At the same time, it is at any level some fragment of the past (even coming from the present) and that a really important point to have in mind.

It is a problematic that keeps going on in history and their associates all the time. In some more objective way, like any other science, source is reliable from your objectives and problems during the research. So, a good theory basis and methodological principles are so important to define when we start some research. When we know the questions, we can answer that better.

If you really want to understand more about this, I suggest to read some historiography texts from Marc Bloch (what is pretty basic but necessary), Fernand Braudel, Friedrich Nietzsche (yep, some texts could really help), Walter Benjamin, Michel Foucault and Paul Veyne. After that, you could go to some Reinhart Koselleck and Paul Ricouer historiography texts.

Determining the validity of sources comes from a variety of methods. Depending on where the source comes from (the source of the source, haha) like if it's a primary source or a secondary source is important. If a source is primary, ie written during the period the event(s) happened, we can determine validity using a number of methods.

1. Archaeology. If a source states that clay pots were constructed from x river, and yet the clay samples fit with another river, the source may not be valid.

2. Comparing with other primary sources. Often, numerous sources state entirely different things about how events took place. However, if there are numerous works from the same period saying that the same reasonable thing happened, it's more likely that that happened then something else.

If a source is secondary, ie written after the fact and using primary sources or archaeological evidence, we can determine validity by looking at the authors past history. Do they have a history of making false claims? Have they written noted works? Were they limited by censors or the context of the work? By far the best way to determine the validity of a secondary source is looking at the sources they cited. By looking at the secondary sources and the primary sources they cited, we can go back to observing the quaity of the primary sources and make assumptions about the quality of the secondary source.

Unreliable sources, especially primary ones, are often just as useful as reliable ones. Whenever looking at a historical work, historians must consider the context of the source. Was it written by someone in an oppressive government that limited free speech and slanted a certain direction? Were writers of the time period predisposed to use unreliable sources or take stories as truth? Did the writer have a financial incentive to include certain details or use certain language? Often, historians can gleam just as much about the writer and their time as about the time they are writing about. Before the birth of modern historical practice, historical writing is often mixed with myths and stories. That’s not a downside; it’s part of learning about how the event was perceived.

This is just some of what could be said. If anybody else has anything to add, please do.