Ever since I began to think of poker as a game of strategy instead of a game of luck I've viewed things similar to the way Euph describes. I am my baseline. I know what I would do, what I consider good poker. As I develop my poker knowledge and skill that baseline changes, but I still see my opponents in comparison to my own mindset.
Mainly I use this in note-taking. Why take notes on lots of hands that are played well, played exactly the way you would play them? Simply write something like "plays solid xx type hands, good positional play, good raising..." or whatever. The vaste majority of my notes are about things that I see people do that deviate from my understanding of good poker "too tight, too aggressive, bluffs too much..." etc.
So what does this have to do with this discussion? As we sit down with unknowns, I view them as a blank slate equal to me. As I note differences, I adjust my view of them, tightening or loosening their ranges based on the factors mentioned above (% of hands played, % of times coming in for raise, bet sizing, etc.).