I never thought of myself as a guy who watches procedural shows but apparently I am. Elementary, Bones, Castle, Forever (ie. ersatz Castle), Backstrom, Unforgettable proves me otherwise.
  1. The killer is usually introduced in the first 15 minutes.
    Pay attention to all character introduced in the 1st act of the show.
  2. If it is an actor you recognize, no matter how small a part he/she had, that character is the lead contender.
    Wait. I think that person had a role as the wife's best friend on the show that was cancelled after 3 episode four years ago that starred Matthew Perry in his first blinked-and-you-missed-it post-Friends appearance. Since I recognize the actor, I'm moving that character to the top of the list. Also, the more recognizable the actor, the more likely he/she is the killer. If it's Jeff Goldblum, he's the killer.
  3. If they bring the character in for questioning and they ask for a lawyer you can eliminate them from the short list.
    There's occasionally a fake out but usually if they ASK for a lawyer, it's not the killer. If they BRING a lawyer with them to the questioning though, they're still in the game. In a strange bizzaro TV land, only innocent people on TV shows ask for lawyers.
  4. If they confess, obviously they are the killer.
    At this point it's too late to be all "I knew it was Jeff Goldblum!" But let's face it. The killer always confesses. They never ask for a lawyer. I don't knew why that is. If I were a killer I'd totally ask for a lawyer and not just confess. That's plain stupid. But not in TV land!
  5. Unless they confess with 15 minutes left on the show!
    Fake out! Confessing in the 3rd act of the show is a clear red herring. In the real world the cops would probably be all "case closed" but in TV land, the sharp and eagle eyed cop/partner/assistant/superhero/investigator/housewife-that-solves-crime-in-her-spare-time will KNOW something is suspicious about the confession and despite everyone else telling him/her to drop it, they will investigate deeper to find the real killer.
  6. If it's a show from the 80s (Murder She Wrote, Hart to Hart, Magnum P.I.) where the crime solvers are NOT cops, the killer is usually a distant friend or relative the main character hasn't seen in years.
    This really only applies to old 80s shows but it's worth noting if you happen to be watching them because nothing else is on. Why and how sweet old Jessica Fletcher became friends with so many morally corrupt killers is beyond me.
  7. It's usually not the 1st person they bring in to question. Too easy. Unless it's a fake out (rare but happens).
    Start scrutinizing the 2nd or 3rd suspect closely. Those are immediately more suspicious. Once you get to the 4th and 5th suspect though it's too late in the game and it's probably going to loop back to 2nd or 3rd. Or someone they didn't suspect at all (the victims co-worker without a seeming motive but actually was sleeping with the victim's significant other).
  8. Note anything unusual that they focus on in scenes with suspects. Characters with noticeable quirks are usually killers.
    If they meet a suspect and the camera pans to a weird stuffed raccoon behind his desk or if she makes a big deal ordering a gluten-free sandwich that's an obvious clue. Inevitably the medical examiner will discover the death had something to do with taxidermy or the body was covered in rice flour and xanthan gum (the binder used in gluten free bread). You'll totally feel superior because you noticed it even though the camera clearly & obviously panned to specifically show that quirk.
  9. Finally, when in doubt, just look for the character with no motive.
    At least no clear motive. Yet. Until the big reveal when the investigators discover a HUGE motive. But usually that's at the very end. If a suspect is introduced with a clear motive for killing the victim from the beginning, usually that's not the killer. Too easy.