I've watched almost 18 series of the BBC Midsomer Murders, and I think only one time did I say, "Ahhh, the butler did it" (or whoever it was). Every other time I tend to keep guessing until the end. I also notice that most of the time right before the big reveal, there's a bit of a cut so they don't reveal whodunnit.
I haven't actually gone back and watched any episodes to see if I could spot where there were bits of evidence... but can you solve the crimes before the detectives, or is it pretty much just a guessing game? I'm not asking if anyone guessed correctly or if I personally missed anything. I'm asking if it is generally possible to deduce the murderer based on the evidence provided to the audience or if it is deliberately kept a secret till the actual reveal.