I go with my gut unless my gut isn't saying anything. Then I run through what I do have memorized that include Thirteen's meaning of the numbers, suits and modes, the tiny bit I know about astrology, the RWS scenes on the minors, the Thoth labels and representations, and what I see right in front of me on the particular card I'm looking at.
For example, I was reading a spread where the "issue" card was a 6 of cups. My gut, linked to my knowledge of the card meanings (I was working with a deck with pips not scenes) said "Past impacting present situation." I noticed there were partial circles in the background and they made me think of turning sommersaults, and the word "tumultuous" came to mind and was a very fitting elaboration of the card meaning in that position.