S - Source
Where did it come from? Who wrote it, published it, posted it? When was it written?
M - Motivation
What are they trying to sell you? Who has what to gain (or lose)?
Where'd they get their data? Are there citations? Who is (or is not) quoted?
L - Logic
Can you follow their train of thought? Do they commit logical fallacies?
L- Left Out
What aren't they telling you?