Can we trust the source?
I recommend using SHEG's (Stanford Education Group) lesson on evaluating sources (Register for free to download an awesome lesson). Click here to download their free lesson on evaluating historical sources (good for all grade levels).
In addition, use this free Google Presentation (and activity) for students to complete by themselves or in groups. You can print as placards and students write on a piece of paper which source is "stronger" than the other and explain why they think so.
Below: sample image from the presentation: