I am a fan of British TV mysteries, many of which have made it to PBS in the U.S.: Sherlock Holmes, Miss Marple, Poirot, Morse, Inspector Lynley, Lewis, et al.
These are all whodunnits -- in that the murderer's identity is unknown to the detectives, be they police or amateurs, and to the viewers. Clues are dropped along the way so that viewers can figure it out, but there is usually enough of a suspect pool and red herrings scattered about so you you have to pay attention before the credits finish or you may miss a major pointer.
This is all fun, and as a writer, I always enjoy looking for the small bits of business that lead to the unraveling of the mystery. Why was the cell phone on the car seat? What did that clipping in the maid's drawer about the war mean? That dusty photograph in the pages of an old book? You know it meant something. Start trying to tie it to what comes later.
For those of you who enjoy such things, too, and who haven't figured out one of the long-standing tropes and major clues, here's a cheat for you:
If you are allowed glimpses of the killing(s), often more than one as the episode goes on, and due to the light, angle, or clothing the perpetrator wears, you cannot tell the sex of the slayer?
Nine times out of ten, the killer will be revealed as a woman.
A hat pulled low and gloves are giveaways; and there will almost alway be a couple-three women it could be. Generally, it will be the one with the fewest apparent reasons for the crime.
A good writer plays relatively fair, but they sometimes skew things. The killer is never the obvious first person they arrest, even if he confesses. He didn't do it, and he confessed for what he considers his own good reasons.
Sometimes the twists and turns will be Byzantine and intricately complex; the most recent Lewis (Inspector Lewis, in the U.S. version) episode I watched "Life Born of Fire," was a doozy, but the clues were fair -- even though I missed a big one. I figured out who the killer was, and why, but not how. Give that one to the writer, Tom MacRae -- it was cleverly done. In retrospect, I wanted to shake my head. Of course. Why didn't I see that?
Well, because they generated a lot of smoke and flashed mirrors to keep me from seeing it, that's why. But still ..
But: if you cannot tell that the killer is a man or a woman when s/he garrotes the usually-deserving victim?
The killer is almost certainly a woman. Impress your spouse with this, and thank me later.