How can historians be more critical about sources?
First of all, I would argue that historians are generally quite good at being critical about sources.
Assuming, though, that historians need to be more critical about sources, the way to do so is to think more thoroughly about the potential biases that may be behind a given text. Historians need to think about the context in which each source they use was created. They need to think about what assumptions the author of the text might have had in mind. They need to determine how these assumptions could make the source less objective. This does not mean that they cannot use these sources. All sources have value, but historians need to be very aware of the potential biases in sources and they need to take those biases into account when using the sources.