How clues signal an abbreviation
Crossword convention says an abbreviated answer must be flagged in the clue. Watch for tags like "Abbr.", "for short", "briefly", "informally" - or an abbreviation used inside the clue itself ("U.S. org." wants an abbreviated agency). See one of these and you can rule out full words immediately.
Compass points
The all-time grid fillers. Any clue like "Opposite of WSW" or "Miami-to-Boston dir." wants a three-letter compass point: ENE, ESE, NNE, NNW, SSE, SSW, WNW, WSW. You only need the first letter from a crossing to lock in most of it.
Ranks and the military
GEN (general), SGT (sergeant), COL (colonel), MAJ (major), CPL (corporal), ENS (ensign), plus NCO and USO. "Mil. rank" or "Army fig." clues almost always land here.
Degrees and school
BA, BS, MA, MBA, PHD, MD, JD - clued as "Grad's deg." or "C.E.O.'s deg.". Add PROF, ELEM, and EDU for campus-flavored clues.
Time, measures, and Latin
YRS, HRS, MIN, SEC for time; LBS, OZS, TSP, TBSP in the kitchen; ETC, ETAL, IBID from Latin. "And others" is ETAL so often it's practically a reflex.
Sports and everything else
RBI, ERA, TKO, KO, MVP, REF cover the sports page (note ERA pulls double duty as a real word - "Pitching stat" vs "Historical period"). CPU, USB, ISP, URL handle tech. When a clue names an organization - "Space org." (NASA), "D.C. baseball team" (NATS) - the answer is usually the abbreviation everyone actually uses in speech.
The takeaway
Abbreviations feel like trivia, but they're a closed set: a few dozen cover the vast majority of puzzles. Skim this list twice and you'll start filling them from the clue alone - which turns their crossings into free letters. Pair this with the crosswordese glossary and short-answer grids lose most of their teeth.