Hmm.  A friend of mine in Zimbabwe got arrested for insulting a police officer at a demo outside the High Court.  All he did was alert some photographers that someone was being arrested, and the police grabbed him and tossed him in the van too.  He spent quite a few days in jail, but eventually they had to release him without charge because his lawyer had collected statements from a bunch of witnesses who were willing to testify that he had never insulted or even spoken to the police officers.  The witnesses included a High Court judge, who'd watched the whole thing from his office window.

But absent the testimony of a judge in your favor, yeah, it would probably be your word against the cop's.  At which point you'd be the first person I know to be convicted of sneering.