What factors can I use to evaluate an opinion presented in an historical source?

It's important to understand initially that on account of being human, no writer will be free completely from bias. Even if they insist otherwise, writers (as with everyone else!) take a particular viewpoint on a subject influenced by their life. Once we understand this, we can start looking at factors specific to a certain writer and judge these against our own knowledge to form an evaluation. This evaluation comes in the form of a comparison between what we do or do not know about a subject and what the author both states and implies.
From a starting point, often the most obvious piece of information you can glean is the date that something was written, if provided. It's important to take into account absolutely everything on the page including dates and locations if provided. This can lead to comparison with personal knowledge on a matter. Similarly, writing style, opinions proffered, class, race, age and many more factors can provide in depth evaluation when combined. It is important therefore to note all of these points down before trying to structure an answer. Finally, what is not on the page is as important as what is. If something you know to be true is omitted for example, you can begin to ask why that is the case. Answering source questions comes down to what questions you can ask of the source, the whys, hows, and why nots that it presents to you. With an open mind, any piece of information can be used to your advantage here.