Lateral reading is a method of investigating the source. It means that you don't rely only on the source you're evaluating, but branch out to use other resources like Wikipedia or using a search engine to find more information on the source itself.
The videos below demonstrate lateral reading.
Determining bias in a source can be challenging because it isn't always obvious; bias can be very subtle. There also isn't a single step to take, but the steps that you take to evaluate a source can help you identify bias.
The following guide from the University of Wisconsin-Green Bay Libraries lays out some excellent steps if you're having trouble.