First off, apply the "4 Ws":
WHO wrote/ produced it?
WHEN was it produced?
WHERE was it produced?
WHY was it produced?
This provides a good basic framework for assessment of provenance, utility & reliability of a source.
If you can't readily answer any of the above, that itself may be significant - eg why might a source be anonymous?
Location of source production may be very telling - eg a Russian writing in W Europe/ N America re 1917: an exile? If so, why? etc etc.
For what audience was the source intended? Private correspondence; official report; electioneering/ political propaganda; intelligence report; published memoir; newspaper article? etc etc. The purpose/ intent of the source, and the relevant conventions of linguistic usage etc, will almost invariably have influenced its linguistic style, symbolism etc., plus - of course - what is/ is not revealed!
One could go on and on - the possible variables are endless...
Remember - always question; always be sceptical; never accept sources at face value, and always seek corroboration of one source by others. Ask yourself how typical/ representative is this source? Don't forget that the exceptional, freakish even, tends to be recorded (esp true of news reports, personal recollections etc) whilst the everyday/ unremarkable/ "normal" stuff gets overlooked - precisely because it's ordinary, "normal", and boring!
Look for IMPLICIT as well as explicit meaning: eg if a propaganda poster has been produced, esp if known to have been widely disseminated, then that points to a significant level of organised activity; access to print technology/ technical support; some level of funding (very probably!); quite sophisticated planning, coordination etc..
Another example of implicit meaning might be what is omitted from a document/ statement/ set of minutes or whatever: the "dog whistle" principle. The minutes of the Wannsee Conference being a good case in point - no explicit mention of mass murder of Jews, but plentiful pointed euphemism - "special action" etc..
Primary sources are not intrinsically more useful or reliable than secondary ones: again, depends on many factors, not least what exactly you want to know about! A good historian who has undertaken a thorough review of all major sources, and read many other historians' opinions/ interpretations, and who is emotionally detached from the events described, may provide a more objective account/ analysis (derived from a far broader perspective) than someone who was actually there, and caught up in events. Personal memories can be less than wholly reliable, and often people tend to remember events in ways which reflect creditably upon themselves! Others - Winston Churchill being a good case in point - will tend to remember things as they feel they ought to have been rather than exactly how they probably were! Also, personal recollections, although often invaluable in many ways, do tend to distort things by the narrowness of their focus. So, Auntie Vi's memoirs of sitting out the Blitz down the local tube station may be invaluable for insights into how housewives/ mothers coped etc, but would be bugger all use for ascertaining anything about how the Battle of Britain was conducted at tactical or strategic levels.
Always remember that the camera can, and frequently does, lie! Who took a photo, & why? Was the photo staged? Has it been "doctored"?
Beware statistics! How were they gathered? Methodology? How representative a sample? etc etc. How is the data presented; what's the base line? etc. Don't ever forget that, statistically, it is not untrue to state that all humans each have one breast & one testicle! In the real world, of course, it's bo**okcs!
Never fall into the trap of assuming that a biased/ unreliable source is not useful. In fact, frequently it's the iffy ones that are most useful - eg "Mein Kampf" is hardly a reliable account of anything, let alone the history of the Jews in Europe, but it's essential reading if you want to understand the mind set of Hitler & the ideology of National Socialism. Similarly, Soviet art of the Stalin era is pure propaganda, but provides fantastic insights into the Stalinist ethos/ world view.
At the end of the day, you just have to keep asking questions, and do not make ready assumptions about anything. Think of a source as a dodgy geezer trying to sell you something - do you buy it?!
Hope this is of some use?
Best wishes,
Wessex_Man.
