Collectively, our knowledge of history is shaped through interpretation. When working with primary sources, historians must confront two essential realities:
1. The records of historical events reflect the author(s)/participant(s) personal, social, political, or economic perspectives. No matter how impartially presented an account may appear, essentially these accounts remain subjective.
2. It works both ways. Researchers hold their own biases, shaped by their own personal situations and social environments.
Historians use primary sources as evidence to support their historical arguments. As previously noted, primary sources often reflect the interests and concerns of their creator or author so must be critically examined and evaluated. When using primary sources, try to determine:
Not all primary source materials will contain enough information to sufficiently answer the above but hopefully enough information is available to help you decide how reliable the source is and how you will use it.
Secondary sources allow us to learn how other historians have interpreted primary sources in order to understand an event. It is equally important to evaluate the reliability and validity of secondary sources as much as the primary. Points to consider:
As with primary sources, you may not be able to ascertain all of the above, but you should be able to determine enough about the source in order to determine its usefulness.
Reading a text in isolation can be dangerous. Thinking about evaluating sources as "good" or "bad" is misleading. There is a lot of grey and information is all about CONTEXT.
The way information is created today is interconnected and complicated; context is the key to understanding the credibility of the source. When you incorporate lateral reading into your process of evaluating a source, it encourages you to dig deeper to see the bigger picture and to wonder about how this source is interconnected with others. When you're properly evaluating a source, you're not just wandering around looking for an article to use for your paper, you're wondering about uncovering any bias within the source and how it fits within context of your knowledge, research, and purpose.
Lateral reading means you spend less time on the source you're thinking of using and more time searching for information about the source. Instead of going to the About Us page of a website to learn about the organization you've never heard of (vertical reading), you'd open up a new tab and search for other sources about the organization, it's founders, and it's reputation (lateral reading).
There are plenty of organizations that have a veneer of authenticity and neutrality as a disguise for advocacy or lobbying and plenty of publishers that have leaning agendas that won't ever run a story that doesn't support the narrative they want to portray. Lateral reading encourages you to ask many of the 5 W's (especially who, what, why) with the underlying goal of discovering "what sort of public memory does this information invoke?". These steps help you understand the context of the source, uncover nuances and subtle signaling that only are revealed through critical thinking.
Instead of asking whether this is a "good source" or a "bad source" ... you're asking "why would I use this source" and making your decision based on the context you've uncovered.
Sources:
Civic Online Reasoning (n.d.). Curriculum. https://cor.stanford.edu/curriculum/
Wineburg, S., & McGrew, S. (2017). Lateral reading: Reading less and learning more when evaluating digital information. Stanford History Education Group Working Paper No. 2017-A1, Oct. 9, 2017.

Stop: Understand who created this information and why
Investigate the Source: Know what you are reading before you read it
Find Better or Other Coverage: Look for other reporting or analysis on the claim
Trace Quotes, Media and Claims to Original Source: Understand the original context
Sources:


Currency: the timeliness of the information
Relevance: the importance of the information for your needs
Authority: the source of the information
Accuracy: the reliability, truthfulness, and correctness of the content
Purpose: the reason the information exists