When I look at a source, how do I know what it's trying to say?

When you first look at a source I want you to stop for a second and ask "what is this source showing me" at its most basic level. For example, if it is an image, what is in it? Who is in it? How do they look? What are they doing? (at this point I'd pull out an example relevant to what they had been studying and ask them, most likely from a past paper.). Then I want you to look at the context of the source. Who wrote/published it? What were they trying to achieve? What key events happened around the time this source was produced? How would they have thought about these events? How reliable is it?
After answering those questions I'd check whether or not they'd be comfortable answering the source question. If the answer was no, then I'd continue to ask questions attempting to get them to liink what they saw in the source to the context and to understand the point of view of the person/people who produced the source.