It is important to analyse the author of the source first of all. Each person has varying socio-economic backgrounds or political agendas and these are very important in how you should approach the source as they add a significant degree of bias. If it is a person that is fundamental to your course who you know a significant degree of information about, use that information to add weight to your analysis of the source's usefulness. It is also essential to consider what the intended audience of the source is. Posters might be used so that many people can see the information, but letters are clearly meant for more personal readership. The list of possible sources is endless, but by considering the intended audience, one can go a long way to evaluating the source's true meaning.