How can I evaluate sources in my essays?

The best way to evaluate sources is to go back to the basics: Who, What, When, Where and Why?

Who wrote it and are they likely to be biased?

What have they written and does it agree with what you have read before? Do they use inflammatory language? 

When was it written? At the time of the event or a long time afterwards?

Where was it written? If it's about Clemanceau and was written in Germany is it likely to be biased?

Why was it written? If it was written as propaganda to support the British war effort in World War II is it likely to be an accurate, reliable source?

It may sound obvious and a lot like GCSE but these five points are what you need to have in your head as a basic structure for evaluating sources. Throw a little cross-referencing of sources and (if you're feeling smart) some other historian's points of view in there and you'll be sailing.