The Sneakoscope given to Harry by Ron in PoA seems to work. Though it goes off sometimes when nothing apparently untrustworthy is happening. For instance on page 72 US version of PoA it goes off on the Hogwarts express. Ron, Harry, Hermione and a sleeping Lupin are the only ones in the compartment. Harry just told them that Black was after them. That doesn’t seem very untrustworthy. The only thing that I can think of is that Lupin was evesdropping on the conversation and only pretending to be asleep. Or it was just wrong. But what do you think?