Step 1 — Count the safeties

Your eyes go to the safeties first. Two safeties deep = middle-of-the-field-open (the Cover 2, 4, or 6 family). One safety deep = middle-of-the-field-closed (Cover 1 or 3). No safety deep = Cover 0 or an all-out blitz.

Every single snap — this is always read one, before anything else.