When evaluating an information source, it means learning as much as you can about a source from other sources. Reading laterally involves doing research about the information you have found, and WAAC defines 4 levels to investigate.
Information evaluation is not easy and it is becoming increasingly complicated when it comes to online information.
Please keep in mind that this is just one set of strategies.
There are many more considerations and strategies to consider, and your approach will depend on the information at hand and your particular information need (how you are using the information).
For instance, here is a sampling of some other evaluation strategies: