How do I evaluate the usefulness/ reliability of a source?

Firstly consider the nature of the source as that affects the broadness of its coverage and its neutrality. For instance, a chronicle or a news report may just simply provide a neutral narrative of what happened whilst a speech or a pamphlet is likely to be very partisan, given that such things are produced to promote a viewpoint to the public, and thus may include selective or even distorted information. Secondly consider who produced the source and consider if they would have had vested interests (i.e. political/ religious affiliations) in advocating for a particular point of view. However, do not dismiss them because of that, but instead consider how their partisan affiliations might affect the reliability of their source - note what selective coverage/ distortion of facts it has created that you can see in the source. Also consider how close the author was to the events, as if they were directly involved or at least present there they are likely to provide valuable insights, thus making their source authoritative, whilst if they were distant from the events then the information they present may be based on gossip, rumour or second hand information, which can be unreliable. Thirdly, consider the date in which it was produced.  If it was produced at the time that the event in question happened, then it might have limited hindsight about the consequences and implications of events, if that's what you are considering, whilst if it was produced some time after then it might be able to offer significant hindsight, which would further its utility.