How can you tell if a source is reliable?

First you can look at the date of the source and if it is the same or almost the same as when the original event happened then it will be more of a primary source. Also if the source has a lot of detail and evidence it will most likely be reliable. You also want to look out for unbiased articles/sources. Having a biased opinion of something can change the way the source is written and effects the source by listing opinions rather than facts.